diff --git a/src/PhpSpreadsheet/Reader/Xlsx/Namespaces.php b/src/PhpSpreadsheet/Reader/Xlsx/Namespaces.php
index 7f484c2f..fa3e57e7 100644
--- a/src/PhpSpreadsheet/Reader/Xlsx/Namespaces.php
+++ b/src/PhpSpreadsheet/Reader/Xlsx/Namespaces.php
@@ -18,23 +18,39 @@ class Namespaces
 
     const THEME = 'http://schemas.openxmlformats.org/package/2006/relationships/theme';
 
+    const THEME2 = 'http://schemas.openxmlformats.org/officeDocument/2006/relationships/theme';
+
     const COMPATIBILITY = 'http://schemas.openxmlformats.org/markup-compatibility/2006';
 
     const MAIN = 'http://schemas.openxmlformats.org/spreadsheetml/2006/main';
 
+    const RELATIONSHIPS_DRAWING = 'http://schemas.openxmlformats.org/officeDocument/2006/relationships/drawing';
+
     const DRAWINGML = 'http://schemas.openxmlformats.org/drawingml/2006/main';
 
     const CHART = 'http://schemas.openxmlformats.org/drawingml/2006/chart';
 
+    const CHART_ALTERNATE = 'http://schemas.microsoft.com/office/drawing/2007/8/2/chart';
+
+    const RELATIONSHIPS_CHART = 'http://schemas.openxmlformats.org/officeDocument/2006/relationships/chart';
+
     const SPREADSHEET_DRAWING = 'http://schemas.openxmlformats.org/drawingml/2006/spreadsheetDrawing';
 
     const SCHEMA_OFFICE_DOCUMENT = 'http://schemas.openxmlformats.org/officeDocument/2006/relationships';
 
     const COMMENTS = 'http://schemas.openxmlformats.org/officeDocument/2006/relationships/comments';
 
-    //const CUSTOM_PROPERTIES = 'http://schemas.openxmlformats.org/officeDocument/2006/relationships/custom-properties';
+    const RELATIONSHIPS_CUSTOM_PROPERTIES = 'http://schemas.openxmlformats.org/officeDocument/2006/relationships/custom-properties';
 
-    //const EXTENDED_PROPERTIES = 'http://schemas.openxmlformats.org/officeDocument/2006/relationships/extended-properties';
+    const RELATIONSHIPS_EXTENDED_PROPERTIES = 'http://schemas.openxmlformats.org/officeDocument/2006/relationships/extended-properties';
+
+    const RELATIONSHIPS_CTRLPROP = 'http://schemas.openxmlformats.org/officeDocument/2006/relationships/ctrlProp';
+
+    const CUSTOM_PROPERTIES = 'http://schemas.openxmlformats.org/officeDocument/2006/custom-properties';
+
+    const EXTENDED_PROPERTIES = 'http://schemas.openxmlformats.org/officeDocument/2006/extended-properties';
+
+    const PROPERTIES_VTYPES = 'http://schemas.openxmlformats.org/officeDocument/2006/docPropsVTypes';
 
     const HYPERLINK = 'http://schemas.openxmlformats.org/officeDocument/2006/relationships/hyperlink';
 
@@ -58,13 +74,29 @@ class Namespaces
 
     const VBA = 'http://schemas.microsoft.com/office/2006/relationships/vbaProject';
 
+    const VBA_SIGNATURE = 'http://schemas.microsoft.com/office/2006/relationships/vbaProject';
+
     const DATA_VALIDATIONS1 = 'http://schemas.microsoft.com/office/spreadsheetml/2009/9/main';
 
     const DATA_VALIDATIONS2 = 'http://schemas.microsoft.com/office/excel/2006/main';
 
+    const CONTENT_TYPES = 'http://schemas.openxmlformats.org/package/2006/content-types';
+
+    const RELATIONSHIPS_PRINTER_SETTINGS = 'http://schemas.openxmlformats.org/officeDocument/2006/relationships/printerSettings';
+
+    const RELATIONSHIPS_TABLE = 'http://schemas.openxmlformats.org/officeDocument/2006/relationships/table';
+
+    const SPREADSHEETML_AC = 'http://schemas.microsoft.com/office/spreadsheetml/2009/9/ac';
+
     const DC_ELEMENTS = 'http://purl.org/dc/elements/1.1/';
 
-    const DC_TERMS = 'http://purl.org/dc/terms';
+    const DC_TERMS = 'http://purl.org/dc/terms/';
+
+    const DC_DCMITYPE = 'http://purl.org/dc/dcmitype/';
+
+    const SCHEMA_INSTANCE = 'http://www.w3.org/2001/XMLSchema-instance';
+
+    const URN_EXCEL = 'urn:schemas-microsoft-com:office:excel';
 
     const URN_MSOFFICE = 'urn:schemas-microsoft-com:office:office';
 
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Chart.php b/src/PhpSpreadsheet/Writer/Xlsx/Chart.php
index 3b64bd8e..5bb12948 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Chart.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Chart.php
@@ -12,6 +12,7 @@ use PhpOffice\PhpSpreadsheet\Chart\PlotArea;
 use PhpOffice\PhpSpreadsheet\Chart\Properties;
 use PhpOffice\PhpSpreadsheet\Chart\Title;
 use PhpOffice\PhpSpreadsheet\Chart\TrendLine;
+use PhpOffice\PhpSpreadsheet\Reader\Xlsx\Namespaces;
 use PhpOffice\PhpSpreadsheet\Shared\XMLWriter;
 use PhpOffice\PhpSpreadsheet\Writer\Exception as WriterException;
 
@@ -48,9 +49,9 @@ class Chart extends WriterPart
 
         // c:chartSpace
         $objWriter->startElement('c:chartSpace');
-        $objWriter->writeAttribute('xmlns:c', 'http://schemas.openxmlformats.org/drawingml/2006/chart');
-        $objWriter->writeAttribute('xmlns:a', 'http://schemas.openxmlformats.org/drawingml/2006/main');
-        $objWriter->writeAttribute('xmlns:r', 'http://schemas.openxmlformats.org/officeDocument/2006/relationships');
+        $objWriter->writeAttribute('xmlns:c', Namespaces::CHART);
+        $objWriter->writeAttribute('xmlns:a', Namespaces::DRAWINGML);
+        $objWriter->writeAttribute('xmlns:r', Namespaces::SCHEMA_OFFICE_DOCUMENT);
 
         $objWriter->startElement('c:date1904');
         $objWriter->writeAttribute('val', '0');
@@ -1553,11 +1554,11 @@ class Chart extends WriterPart
     private function writeAlternateContent(XMLWriter $objWriter): void
     {
         $objWriter->startElement('mc:AlternateContent');
-        $objWriter->writeAttribute('xmlns:mc', 'http://schemas.openxmlformats.org/markup-compatibility/2006');
+        $objWriter->writeAttribute('xmlns:mc', Namespaces::COMPATIBILITY);
 
         $objWriter->startElement('mc:Choice');
         $objWriter->writeAttribute('Requires', 'c14');
-        $objWriter->writeAttribute('xmlns:c14', 'http://schemas.microsoft.com/office/drawing/2007/8/2/chart');
+        $objWriter->writeAttribute('xmlns:c14', Namespaces::CHART_ALTERNATE);
 
         $objWriter->startElement('c14:style');
         $objWriter->writeAttribute('val', '102');
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Comments.php b/src/PhpSpreadsheet/Writer/Xlsx/Comments.php
index 5045e8f3..91973884 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Comments.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Comments.php
@@ -4,6 +4,7 @@ namespace PhpOffice\PhpSpreadsheet\Writer\Xlsx;
 
 use PhpOffice\PhpSpreadsheet\Cell\Coordinate;
 use PhpOffice\PhpSpreadsheet\Comment;
+use PhpOffice\PhpSpreadsheet\Reader\Xlsx\Namespaces;
 use PhpOffice\PhpSpreadsheet\Shared\XMLWriter;
 
 class Comments extends WriterPart
@@ -40,7 +41,7 @@ class Comments extends WriterPart
 
         // comments
         $objWriter->startElement('comments');
-        $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/spreadsheetml/2006/main');
+        $objWriter->writeAttribute('xmlns', Namespaces::MAIN);
 
         // Loop through authors
         $objWriter->startElement('authors');
@@ -107,9 +108,9 @@ class Comments extends WriterPart
 
         // xml
         $objWriter->startElement('xml');
-        $objWriter->writeAttribute('xmlns:v', 'urn:schemas-microsoft-com:vml');
-        $objWriter->writeAttribute('xmlns:o', 'urn:schemas-microsoft-com:office:office');
-        $objWriter->writeAttribute('xmlns:x', 'urn:schemas-microsoft-com:office:excel');
+        $objWriter->writeAttribute('xmlns:v', Namespaces::URN_VML);
+        $objWriter->writeAttribute('xmlns:o', Namespaces::URN_MSOFFICE);
+        $objWriter->writeAttribute('xmlns:x', Namespaces::URN_EXCEL);
 
         // o:shapelayout
         $objWriter->startElement('o:shapelayout');
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/ContentTypes.php b/src/PhpSpreadsheet/Writer/Xlsx/ContentTypes.php
index acb85b57..73657fc3 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/ContentTypes.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/ContentTypes.php
@@ -2,6 +2,7 @@
 
 namespace PhpOffice\PhpSpreadsheet\Writer\Xlsx;
 
+use PhpOffice\PhpSpreadsheet\Reader\Xlsx\Namespaces;
 use PhpOffice\PhpSpreadsheet\Shared\File;
 use PhpOffice\PhpSpreadsheet\Shared\XMLWriter;
 use PhpOffice\PhpSpreadsheet\Spreadsheet;
@@ -32,7 +33,7 @@ class ContentTypes extends WriterPart
 
         // Types
         $objWriter->startElement('Types');
-        $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/package/2006/content-types');
+        $objWriter->writeAttribute('xmlns', Namespaces::CONTENT_TYPES);
 
         // Theme
         $this->writeOverrideContentType($objWriter, '/xl/theme/theme1.xml', 'application/vnd.openxmlformats-officedocument.theme+xml');
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/DefinedNames.php b/src/PhpSpreadsheet/Writer/Xlsx/DefinedNames.php
index 0ca9f64f..24752c9e 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/DefinedNames.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/DefinedNames.php
@@ -8,7 +8,7 @@ use PhpOffice\PhpSpreadsheet\Cell\Coordinate;
 use PhpOffice\PhpSpreadsheet\DefinedName;
 use PhpOffice\PhpSpreadsheet\Shared\XMLWriter;
 use PhpOffice\PhpSpreadsheet\Spreadsheet;
-use PhpOffice\PhpSpreadsheet\Worksheet\Worksheet;
+use PhpOffice\PhpSpreadsheet\Worksheet\Worksheet as ActualWorksheet;
 
 class DefinedNames
 {
@@ -98,7 +98,7 @@ class DefinedNames
     /**
      * Write Defined Name for autoFilter.
      */
-    private function writeNamedRangeForAutofilter(Worksheet $worksheet, int $worksheetId = 0): void
+    private function writeNamedRangeForAutofilter(ActualWorksheet $worksheet, int $worksheetId = 0): void
     {
         // NamedRange for autoFilter
         $autoFilterRange = $worksheet->getAutoFilter()->getRange();
@@ -112,7 +112,7 @@ class DefinedNames
             $range = Coordinate::splitRange($autoFilterRange);
             $range = $range[0];
             //    Strip any worksheet ref so we can make the cell ref absolute
-            [, $range[0]] = Worksheet::extractSheetTitle($range[0], true);
+            [, $range[0]] = ActualWorksheet::extractSheetTitle($range[0], true);
 
             $range[0] = Coordinate::absoluteCoordinate($range[0]);
             if (count($range) > 1) {
@@ -129,7 +129,7 @@ class DefinedNames
     /**
      * Write Defined Name for PrintTitles.
      */
-    private function writeNamedRangeForPrintTitles(Worksheet $worksheet, int $worksheetId = 0): void
+    private function writeNamedRangeForPrintTitles(ActualWorksheet $worksheet, int $worksheetId = 0): void
     {
         // NamedRange for PrintTitles
         if ($worksheet->getPageSetup()->isColumnsToRepeatAtLeftSet() || $worksheet->getPageSetup()->isRowsToRepeatAtTopSet()) {
@@ -167,7 +167,7 @@ class DefinedNames
     /**
      * Write Defined Name for PrintTitles.
      */
-    private function writeNamedRangeForPrintArea(Worksheet $worksheet, int $worksheetId = 0): void
+    private function writeNamedRangeForPrintArea(ActualWorksheet $worksheet, int $worksheetId = 0): void
     {
         // NamedRange for PrintArea
         if ($worksheet->getPageSetup()->isPrintAreaSet()) {
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/DocProps.php b/src/PhpSpreadsheet/Writer/Xlsx/DocProps.php
index 02259c09..8902826a 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/DocProps.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/DocProps.php
@@ -3,6 +3,7 @@
 namespace PhpOffice\PhpSpreadsheet\Writer\Xlsx;
 
 use PhpOffice\PhpSpreadsheet\Document\Properties;
+use PhpOffice\PhpSpreadsheet\Reader\Xlsx\Namespaces;
 use PhpOffice\PhpSpreadsheet\Shared\Date;
 use PhpOffice\PhpSpreadsheet\Shared\XMLWriter;
 use PhpOffice\PhpSpreadsheet\Spreadsheet;
@@ -29,8 +30,8 @@ class DocProps extends WriterPart
 
         // Properties
         $objWriter->startElement('Properties');
-        $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/officeDocument/2006/extended-properties');
-        $objWriter->writeAttribute('xmlns:vt', 'http://schemas.openxmlformats.org/officeDocument/2006/docPropsVTypes');
+        $objWriter->writeAttribute('xmlns', Namespaces::EXTENDED_PROPERTIES);
+        $objWriter->writeAttribute('xmlns:vt', Namespaces::PROPERTIES_VTYPES);
 
         // Application
         $objWriter->writeElement('Application', 'Microsoft Excel');
@@ -124,11 +125,11 @@ class DocProps extends WriterPart
 
         // cp:coreProperties
         $objWriter->startElement('cp:coreProperties');
-        $objWriter->writeAttribute('xmlns:cp', 'http://schemas.openxmlformats.org/package/2006/metadata/core-properties');
-        $objWriter->writeAttribute('xmlns:dc', 'http://purl.org/dc/elements/1.1/');
-        $objWriter->writeAttribute('xmlns:dcterms', 'http://purl.org/dc/terms/');
-        $objWriter->writeAttribute('xmlns:dcmitype', 'http://purl.org/dc/dcmitype/');
-        $objWriter->writeAttribute('xmlns:xsi', 'http://www.w3.org/2001/XMLSchema-instance');
+        $objWriter->writeAttribute('xmlns:cp', Namespaces::CORE_PROPERTIES2);
+        $objWriter->writeAttribute('xmlns:dc', Namespaces::DC_ELEMENTS);
+        $objWriter->writeAttribute('xmlns:dcterms', Namespaces::DC_TERMS);
+        $objWriter->writeAttribute('xmlns:dcmitype', Namespaces::DC_DCMITYPE);
+        $objWriter->writeAttribute('xmlns:xsi', Namespaces::SCHEMA_INSTANCE);
 
         // dc:creator
         $objWriter->writeElement('dc:creator', $spreadsheet->getProperties()->getCreator());
@@ -198,8 +199,8 @@ class DocProps extends WriterPart
 
         // cp:coreProperties
         $objWriter->startElement('Properties');
-        $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/officeDocument/2006/custom-properties');
-        $objWriter->writeAttribute('xmlns:vt', 'http://schemas.openxmlformats.org/officeDocument/2006/docPropsVTypes');
+        $objWriter->writeAttribute('xmlns', Namespaces::CUSTOM_PROPERTIES);
+        $objWriter->writeAttribute('xmlns:vt', Namespaces::PROPERTIES_VTYPES);
 
         foreach ($customPropertyList as $key => $customProperty) {
             $propertyValue = $spreadsheet->getProperties()->getCustomPropertyValue($customProperty);
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Drawing.php b/src/PhpSpreadsheet/Writer/Xlsx/Drawing.php
index 7693c72c..d4f7f11f 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Drawing.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Drawing.php
@@ -3,6 +3,7 @@
 namespace PhpOffice\PhpSpreadsheet\Writer\Xlsx;
 
 use PhpOffice\PhpSpreadsheet\Cell\Coordinate;
+use PhpOffice\PhpSpreadsheet\Reader\Xlsx\Namespaces;
 use PhpOffice\PhpSpreadsheet\Shared\Drawing as SharedDrawing;
 use PhpOffice\PhpSpreadsheet\Shared\XMLWriter;
 use PhpOffice\PhpSpreadsheet\Spreadsheet;
@@ -34,8 +35,8 @@ class Drawing extends WriterPart
 
         // xdr:wsDr
         $objWriter->startElement('xdr:wsDr');
-        $objWriter->writeAttribute('xmlns:xdr', 'http://schemas.openxmlformats.org/drawingml/2006/spreadsheetDrawing');
-        $objWriter->writeAttribute('xmlns:a', 'http://schemas.openxmlformats.org/drawingml/2006/main');
+        $objWriter->writeAttribute('xmlns:xdr', Namespaces::SPREADSHEET_DRAWING);
+        $objWriter->writeAttribute('xmlns:a', Namespaces::DRAWINGML);
 
         // Loop through images and write drawings
         $i = 1;
@@ -159,10 +160,10 @@ class Drawing extends WriterPart
 
         $objWriter->startElement('a:graphic');
         $objWriter->startElement('a:graphicData');
-        $objWriter->writeAttribute('uri', 'http://schemas.openxmlformats.org/drawingml/2006/chart');
+        $objWriter->writeAttribute('uri', Namespaces::CHART);
         $objWriter->startElement('c:chart');
-        $objWriter->writeAttribute('xmlns:c', 'http://schemas.openxmlformats.org/drawingml/2006/chart');
-        $objWriter->writeAttribute('xmlns:r', 'http://schemas.openxmlformats.org/officeDocument/2006/relationships');
+        $objWriter->writeAttribute('xmlns:c', Namespaces::CHART);
+        $objWriter->writeAttribute('xmlns:r', Namespaces::SCHEMA_OFFICE_DOCUMENT);
         $objWriter->writeAttribute('r:id', 'rId' . $relationId);
         $objWriter->endElement();
         $objWriter->endElement();
@@ -265,7 +266,7 @@ class Drawing extends WriterPart
 
             // a:blip
             $objWriter->startElement('a:blip');
-            $objWriter->writeAttribute('xmlns:r', 'http://schemas.openxmlformats.org/officeDocument/2006/relationships');
+            $objWriter->writeAttribute('xmlns:r', Namespaces::SCHEMA_OFFICE_DOCUMENT);
             $objWriter->writeAttribute('r:embed', 'rId' . $relationId);
             $objWriter->endElement();
 
@@ -362,9 +363,9 @@ class Drawing extends WriterPart
 
         // xml
         $objWriter->startElement('xml');
-        $objWriter->writeAttribute('xmlns:v', 'urn:schemas-microsoft-com:vml');
-        $objWriter->writeAttribute('xmlns:o', 'urn:schemas-microsoft-com:office:office');
-        $objWriter->writeAttribute('xmlns:x', 'urn:schemas-microsoft-com:office:excel');
+        $objWriter->writeAttribute('xmlns:v', Namespaces::URN_VML);
+        $objWriter->writeAttribute('xmlns:o', Namespaces::URN_MSOFFICE);
+        $objWriter->writeAttribute('xmlns:x', Namespaces::URN_EXCEL);
 
         // o:shapelayout
         $objWriter->startElement('o:shapelayout');
@@ -558,7 +559,7 @@ class Drawing extends WriterPart
         }
 
         $objWriter->startElement('a:hlinkClick');
-        $objWriter->writeAttribute('xmlns:r', 'http://schemas.openxmlformats.org/officeDocument/2006/relationships');
+        $objWriter->writeAttribute('xmlns:r', Namespaces::SCHEMA_OFFICE_DOCUMENT);
         $objWriter->writeAttribute('r:id', 'rId' . $hlinkClickId);
         $objWriter->endElement();
     }
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Rels.php b/src/PhpSpreadsheet/Writer/Xlsx/Rels.php
index 99fa2d34..e621dc3e 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Rels.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Rels.php
@@ -2,6 +2,7 @@
 
 namespace PhpOffice\PhpSpreadsheet\Writer\Xlsx;
 
+use PhpOffice\PhpSpreadsheet\Reader\Xlsx\Namespaces;
 use PhpOffice\PhpSpreadsheet\Shared\XMLWriter;
 use PhpOffice\PhpSpreadsheet\Spreadsheet;
 use PhpOffice\PhpSpreadsheet\Worksheet\BaseDrawing;
@@ -30,7 +31,7 @@ class Rels extends WriterPart
 
         // Relationships
         $objWriter->startElement('Relationships');
-        $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/package/2006/relationships');
+        $objWriter->writeAttribute('xmlns', Namespaces::RELATIONSHIPS);
 
         $customPropertyList = $spreadsheet->getProperties()->getCustomProperties();
         if (!empty($customPropertyList)) {
@@ -38,7 +39,7 @@ class Rels extends WriterPart
             $this->writeRelationship(
                 $objWriter,
                 4,
-                'http://schemas.openxmlformats.org/officeDocument/2006/relationships/custom-properties',
+                Namespaces::RELATIONSHIPS_CUSTOM_PROPERTIES,
                 'docProps/custom.xml'
             );
         }
@@ -47,7 +48,7 @@ class Rels extends WriterPart
         $this->writeRelationship(
             $objWriter,
             3,
-            'http://schemas.openxmlformats.org/officeDocument/2006/relationships/extended-properties',
+            Namespaces::RELATIONSHIPS_EXTENDED_PROPERTIES,
             'docProps/app.xml'
         );
 
@@ -55,7 +56,7 @@ class Rels extends WriterPart
         $this->writeRelationship(
             $objWriter,
             2,
-            'http://schemas.openxmlformats.org/package/2006/relationships/metadata/core-properties',
+            Namespaces::CORE_PROPERTIES,
             'docProps/core.xml'
         );
 
@@ -63,7 +64,7 @@ class Rels extends WriterPart
         $this->writeRelationship(
             $objWriter,
             1,
-            'http://schemas.openxmlformats.org/officeDocument/2006/relationships/officeDocument',
+            Namespaces::OFFICE_DOCUMENT,
             'xl/workbook.xml'
         );
         // a custom UI in workbook ?
@@ -72,7 +73,7 @@ class Rels extends WriterPart
             $this->writeRelationShip(
                 $objWriter,
                 5,
-                'http://schemas.microsoft.com/office/2006/relationships/ui/extensibility',
+                Namespaces::EXTENSIBILITY,
                 is_string($target) ? $target : ''
             );
         }
@@ -102,13 +103,13 @@ class Rels extends WriterPart
 
         // Relationships
         $objWriter->startElement('Relationships');
-        $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/package/2006/relationships');
+        $objWriter->writeAttribute('xmlns', Namespaces::RELATIONSHIPS);
 
         // Relationship styles.xml
         $this->writeRelationship(
             $objWriter,
             1,
-            'http://schemas.openxmlformats.org/officeDocument/2006/relationships/styles',
+            Namespaces::STYLES,
             'styles.xml'
         );
 
@@ -116,7 +117,7 @@ class Rels extends WriterPart
         $this->writeRelationship(
             $objWriter,
             2,
-            'http://schemas.openxmlformats.org/officeDocument/2006/relationships/theme',
+            Namespaces::THEME2,
             'theme/theme1.xml'
         );
 
@@ -124,7 +125,7 @@ class Rels extends WriterPart
         $this->writeRelationship(
             $objWriter,
             3,
-            'http://schemas.openxmlformats.org/officeDocument/2006/relationships/sharedStrings',
+            Namespaces::SHARED_STRINGS,
             'sharedStrings.xml'
         );
 
@@ -134,7 +135,7 @@ class Rels extends WriterPart
             $this->writeRelationship(
                 $objWriter,
                 ($i + 1 + 3),
-                'http://schemas.openxmlformats.org/officeDocument/2006/relationships/worksheet',
+                Namespaces::WORKSHEET,
                 'worksheets/sheet' . ($i + 1) . '.xml'
             );
         }
@@ -144,7 +145,7 @@ class Rels extends WriterPart
             $this->writeRelationShip(
                 $objWriter,
                 ($i + 1 + 3),
-                'http://schemas.microsoft.com/office/2006/relationships/vbaProject',
+                Namespaces::VBA,
                 'vbaProject.bin'
             );
             ++$i; //increment i if needed for an another relation
@@ -183,7 +184,7 @@ class Rels extends WriterPart
 
         // Relationships
         $objWriter->startElement('Relationships');
-        $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/package/2006/relationships');
+        $objWriter->writeAttribute('xmlns', Namespaces::RELATIONSHIPS);
 
         // Write drawing relationships?
         $drawingOriginalIds = [];
@@ -215,7 +216,7 @@ class Rels extends WriterPart
             $this->writeRelationship(
                 $objWriter,
                 $rId,
-                'http://schemas.openxmlformats.org/officeDocument/2006/relationships/drawing',
+                Namespaces::RELATIONSHIPS_DRAWING,
                 $relPath
             );
         }
@@ -227,7 +228,7 @@ class Rels extends WriterPart
                 $this->writeRelationship(
                     $objWriter,
                     '_hyperlink_' . $i,
-                    'http://schemas.openxmlformats.org/officeDocument/2006/relationships/hyperlink',
+                    Namespaces::HYPERLINK,
                     $hyperlink->getUrl(),
                     'External'
                 );
@@ -242,14 +243,14 @@ class Rels extends WriterPart
             $this->writeRelationship(
                 $objWriter,
                 '_comments_vml' . $i,
-                'http://schemas.openxmlformats.org/officeDocument/2006/relationships/vmlDrawing',
+                Namespaces::VML,
                 '../drawings/vmlDrawing' . $worksheetId . '.vml'
             );
 
             $this->writeRelationship(
                 $objWriter,
                 '_comments' . $i,
-                'http://schemas.openxmlformats.org/officeDocument/2006/relationships/comments',
+                Namespaces::COMMENTS,
                 '../comments' . $worksheetId . '.xml'
             );
         }
@@ -260,7 +261,7 @@ class Rels extends WriterPart
             $this->writeRelationship(
                 $objWriter,
                 '_table_' . $i,
-                'http://schemas.openxmlformats.org/officeDocument/2006/relationships/table',
+                Namespaces::RELATIONSHIPS_TABLE,
                 '../tables/table' . $tableRef++ . '.xml'
             );
         }
@@ -271,14 +272,14 @@ class Rels extends WriterPart
             $this->writeRelationship(
                 $objWriter,
                 '_headerfooter_vml' . $i,
-                'http://schemas.openxmlformats.org/officeDocument/2006/relationships/vmlDrawing',
+                Namespaces::VML,
                 '../drawings/vmlDrawingHF' . $worksheetId . '.vml'
             );
         }
 
-        $this->writeUnparsedRelationship($worksheet, $objWriter, 'ctrlProps', 'http://schemas.openxmlformats.org/officeDocument/2006/relationships/ctrlProp');
-        $this->writeUnparsedRelationship($worksheet, $objWriter, 'vmlDrawings', 'http://schemas.openxmlformats.org/officeDocument/2006/relationships/vmlDrawing');
-        $this->writeUnparsedRelationship($worksheet, $objWriter, 'printerSettings', 'http://schemas.openxmlformats.org/officeDocument/2006/relationships/printerSettings');
+        $this->writeUnparsedRelationship($worksheet, $objWriter, 'ctrlProps', Namespaces::RELATIONSHIPS_CTRLPROP);
+        $this->writeUnparsedRelationship($worksheet, $objWriter, 'vmlDrawings', Namespaces::VML);
+        $this->writeUnparsedRelationship($worksheet, $objWriter, 'printerSettings', Namespaces::RELATIONSHIPS_PRINTER_SETTINGS);
 
         $objWriter->endElement();
 
@@ -325,7 +326,7 @@ class Rels extends WriterPart
 
         // Relationships
         $objWriter->startElement('Relationships');
-        $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/package/2006/relationships');
+        $objWriter->writeAttribute('xmlns', Namespaces::RELATIONSHIPS);
 
         // Loop through images and write relationships
         $i = 1;
@@ -340,7 +341,7 @@ class Rels extends WriterPart
                 $this->writeRelationship(
                     $objWriter,
                     $i,
-                    'http://schemas.openxmlformats.org/officeDocument/2006/relationships/image',
+                    Namespaces::IMAGE,
                     '../media/' . $drawing->getIndexedFilename()
                 );
 
@@ -359,7 +360,7 @@ class Rels extends WriterPart
                     $this->writeRelationship(
                         $objWriter,
                         $i++,
-                        'http://schemas.openxmlformats.org/officeDocument/2006/relationships/chart',
+                        Namespaces::RELATIONSHIPS_CHART,
                         '../charts/chart' . ++$chartRef . '.xml'
                     );
                 }
@@ -391,7 +392,7 @@ class Rels extends WriterPart
 
         // Relationships
         $objWriter->startElement('Relationships');
-        $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/package/2006/relationships');
+        $objWriter->writeAttribute('xmlns', Namespaces::RELATIONSHIPS);
 
         // Loop through images and write relationships
         foreach ($worksheet->getHeaderFooter()->getImages() as $key => $value) {
@@ -399,7 +400,7 @@ class Rels extends WriterPart
             $this->writeRelationship(
                 $objWriter,
                 $key,
-                'http://schemas.openxmlformats.org/officeDocument/2006/relationships/image',
+                Namespaces::IMAGE,
                 '../media/' . $value->getIndexedFilename()
             );
         }
@@ -424,7 +425,7 @@ class Rels extends WriterPart
 
         // Relationships
         $objWriter->startElement('Relationships');
-        $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/package/2006/relationships');
+        $objWriter->writeAttribute('xmlns', Namespaces::RELATIONSHIPS);
 
         // Loop through images and write relationships
         foreach ($worksheet->getComments() as $comment) {
@@ -436,7 +437,7 @@ class Rels extends WriterPart
             $this->writeRelationship(
                 $objWriter,
                 $bgImage->getImageIndex(),
-                'http://schemas.openxmlformats.org/officeDocument/2006/relationships/image',
+                Namespaces::IMAGE,
                 '../media/' . $bgImage->getMediaFilename()
             );
         }
@@ -483,7 +484,7 @@ class Rels extends WriterPart
         $this->writeRelationship(
             $objWriter,
             $i,
-            'http://schemas.openxmlformats.org/officeDocument/2006/relationships/hyperlink',
+            Namespaces::HYPERLINK,
             $drawing->getHyperlink()->getUrl(),
             $drawing->getHyperlink()->getTypeHyperlink()
         );
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/RelsRibbon.php b/src/PhpSpreadsheet/Writer/Xlsx/RelsRibbon.php
index 8005207c..e231972c 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/RelsRibbon.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/RelsRibbon.php
@@ -2,6 +2,7 @@
 
 namespace PhpOffice\PhpSpreadsheet\Writer\Xlsx;
 
+use PhpOffice\PhpSpreadsheet\Reader\Xlsx\Namespaces;
 use PhpOffice\PhpSpreadsheet\Shared\XMLWriter;
 use PhpOffice\PhpSpreadsheet\Spreadsheet;
 
@@ -27,13 +28,13 @@ class RelsRibbon extends WriterPart
 
         // Relationships
         $objWriter->startElement('Relationships');
-        $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/package/2006/relationships');
+        $objWriter->writeAttribute('xmlns', Namespaces::RELATIONSHIPS);
         $localRels = $spreadsheet->getRibbonBinObjects('names');
         if (is_array($localRels)) {
             foreach ($localRels as $aId => $aTarget) {
                 $objWriter->startElement('Relationship');
                 $objWriter->writeAttribute('Id', $aId);
-                $objWriter->writeAttribute('Type', 'http://schemas.openxmlformats.org/officeDocument/2006/relationships/image');
+                $objWriter->writeAttribute('Type', Namespaces::IMAGE);
                 $objWriter->writeAttribute('Target', $aTarget);
                 $objWriter->endElement();
             }
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/RelsVBA.php b/src/PhpSpreadsheet/Writer/Xlsx/RelsVBA.php
index 55bcd360..30ed79a3 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/RelsVBA.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/RelsVBA.php
@@ -2,6 +2,7 @@
 
 namespace PhpOffice\PhpSpreadsheet\Writer\Xlsx;
 
+use PhpOffice\PhpSpreadsheet\Reader\Xlsx\Namespaces;
 use PhpOffice\PhpSpreadsheet\Shared\XMLWriter;
 use PhpOffice\PhpSpreadsheet\Spreadsheet;
 
@@ -27,10 +28,10 @@ class RelsVBA extends WriterPart
 
         // Relationships
         $objWriter->startElement('Relationships');
-        $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/package/2006/relationships');
+        $objWriter->writeAttribute('xmlns', Namespaces::RELATIONSHIPS);
         $objWriter->startElement('Relationship');
         $objWriter->writeAttribute('Id', 'rId1');
-        $objWriter->writeAttribute('Type', 'http://schemas.microsoft.com/office/2006/relationships/vbaProjectSignature');
+        $objWriter->writeAttribute('Type', Namespaces::VBA_SIGNATURE);
         $objWriter->writeAttribute('Target', 'vbaProjectSignature.bin');
         $objWriter->endElement();
         $objWriter->endElement();
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/StringTable.php b/src/PhpSpreadsheet/Writer/Xlsx/StringTable.php
index 078f940a..7f623933 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/StringTable.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/StringTable.php
@@ -5,28 +5,26 @@ namespace PhpOffice\PhpSpreadsheet\Writer\Xlsx;
 use PhpOffice\PhpSpreadsheet\Cell\Cell;
 use PhpOffice\PhpSpreadsheet\Cell\DataType;
 use PhpOffice\PhpSpreadsheet\Chart\ChartColor;
+use PhpOffice\PhpSpreadsheet\Reader\Xlsx\Namespaces;
 use PhpOffice\PhpSpreadsheet\RichText\RichText;
 use PhpOffice\PhpSpreadsheet\RichText\Run;
 use PhpOffice\PhpSpreadsheet\Shared\StringHelper;
 use PhpOffice\PhpSpreadsheet\Shared\XMLWriter;
-use PhpOffice\PhpSpreadsheet\Worksheet\Worksheet;
+use PhpOffice\PhpSpreadsheet\Worksheet\Worksheet as ActualWorksheet;
 
 class StringTable extends WriterPart
 {
     /**
      * Create worksheet stringtable.
      *
-     * @param Worksheet $worksheet Worksheet
      * @param string[] $existingTable Existing table to eventually merge with
      *
      * @return string[] String table for worksheet
      */
-    public function createStringTable(Worksheet $worksheet, $existingTable = null)
+    public function createStringTable(ActualWorksheet $worksheet, $existingTable = null)
     {
         // Create string lookup table
         $aStringTable = [];
-        $cellCollection = null;
-        $aFlippedStringTable = null; // For faster lookup
 
         // Is an existing table given?
         if (($existingTable !== null) && is_array($existingTable)) {
@@ -85,7 +83,7 @@ class StringTable extends WriterPart
 
         // String table
         $objWriter->startElement('sst');
-        $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/spreadsheetml/2006/main');
+        $objWriter->writeAttribute('xmlns', Namespaces::MAIN);
         $objWriter->writeAttribute('uniqueCount', (string) count($stringTable));
 
         // Loop through string table
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Style.php b/src/PhpSpreadsheet/Writer/Xlsx/Style.php
index 0442c25d..d59b5b62 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Style.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Style.php
@@ -2,6 +2,7 @@
 
 namespace PhpOffice\PhpSpreadsheet\Writer\Xlsx;
 
+use PhpOffice\PhpSpreadsheet\Reader\Xlsx\Namespaces;
 use PhpOffice\PhpSpreadsheet\Shared\StringHelper;
 use PhpOffice\PhpSpreadsheet\Shared\XMLWriter;
 use PhpOffice\PhpSpreadsheet\Spreadsheet;
@@ -37,7 +38,7 @@ class Style extends WriterPart
         // styleSheet
         $objWriter->startElement('styleSheet');
         $objWriter->writeAttribute('xml:space', 'preserve');
-        $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/spreadsheetml/2006/main');
+        $objWriter->writeAttribute('xmlns', Namespaces::MAIN);
 
         // numFmts
         $objWriter->startElement('numFmts');
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Table.php b/src/PhpSpreadsheet/Writer/Xlsx/Table.php
index a974c6cb..cebdd0fd 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Table.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Table.php
@@ -3,6 +3,7 @@
 namespace PhpOffice\PhpSpreadsheet\Writer\Xlsx;
 
 use PhpOffice\PhpSpreadsheet\Cell\Coordinate;
+use PhpOffice\PhpSpreadsheet\Reader\Xlsx\Namespaces;
 use PhpOffice\PhpSpreadsheet\Shared\XMLWriter;
 use PhpOffice\PhpSpreadsheet\Worksheet\Table as WorksheetTable;
 
@@ -34,7 +35,7 @@ class Table extends WriterPart
 
         $objWriter->startElement('table');
         $objWriter->writeAttribute('xml:space', 'preserve');
-        $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/spreadsheetml/2006/main');
+        $objWriter->writeAttribute('xmlns', Namespaces::MAIN);
         $objWriter->writeAttribute('id', (string) $tableRef);
         $objWriter->writeAttribute('name', $name);
         $objWriter->writeAttribute('displayName', $table->getName() ?: $name);
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Theme.php b/src/PhpSpreadsheet/Writer/Xlsx/Theme.php
index 99177292..b6020c09 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Theme.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Theme.php
@@ -2,6 +2,7 @@
 
 namespace PhpOffice\PhpSpreadsheet\Writer\Xlsx;
 
+use PhpOffice\PhpSpreadsheet\Reader\Xlsx\Namespaces;
 use PhpOffice\PhpSpreadsheet\Shared\XMLWriter;
 use PhpOffice\PhpSpreadsheet\Spreadsheet;
 
@@ -121,7 +122,7 @@ class Theme extends WriterPart
 
         // a:theme
         $objWriter->startElement('a:theme');
-        $objWriter->writeAttribute('xmlns:a', 'http://schemas.openxmlformats.org/drawingml/2006/main');
+        $objWriter->writeAttribute('xmlns:a', Namespaces::DRAWINGML);
         $objWriter->writeAttribute('name', 'Office Theme');
 
         // a:themeElements
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Workbook.php b/src/PhpSpreadsheet/Writer/Xlsx/Workbook.php
index 7d08388d..f2a86782 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Workbook.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Workbook.php
@@ -2,6 +2,7 @@
 
 namespace PhpOffice\PhpSpreadsheet\Writer\Xlsx;
 
+use PhpOffice\PhpSpreadsheet\Reader\Xlsx\Namespaces;
 use PhpOffice\PhpSpreadsheet\Shared\Date;
 use PhpOffice\PhpSpreadsheet\Shared\XMLWriter;
 use PhpOffice\PhpSpreadsheet\Spreadsheet;
@@ -32,8 +33,8 @@ class Workbook extends WriterPart
         // workbook
         $objWriter->startElement('workbook');
         $objWriter->writeAttribute('xml:space', 'preserve');
-        $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/spreadsheetml/2006/main');
-        $objWriter->writeAttribute('xmlns:r', 'http://schemas.openxmlformats.org/officeDocument/2006/relationships');
+        $objWriter->writeAttribute('xmlns', Namespaces::MAIN);
+        $objWriter->writeAttribute('xmlns:r', Namespaces::SCHEMA_OFFICE_DOCUMENT);
 
         // fileVersion
         $this->writeFileVersion($objWriter);
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php b/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php
index 75b44706..58259a63 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php
@@ -5,6 +5,7 @@ namespace PhpOffice\PhpSpreadsheet\Writer\Xlsx;
 use PhpOffice\PhpSpreadsheet\Calculation\Information\ErrorValue;
 use PhpOffice\PhpSpreadsheet\Cell\Cell;
 use PhpOffice\PhpSpreadsheet\Cell\Coordinate;
+use PhpOffice\PhpSpreadsheet\Reader\Xlsx\Namespaces;
 use PhpOffice\PhpSpreadsheet\RichText\RichText;
 use PhpOffice\PhpSpreadsheet\Settings;
 use PhpOffice\PhpSpreadsheet\Shared\StringHelper;
@@ -41,15 +42,15 @@ class Worksheet extends WriterPart
         // Worksheet
         $objWriter->startElement('worksheet');
         $objWriter->writeAttribute('xml:space', 'preserve');
-        $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/spreadsheetml/2006/main');
-        $objWriter->writeAttribute('xmlns:r', 'http://schemas.openxmlformats.org/officeDocument/2006/relationships');
+        $objWriter->writeAttribute('xmlns', Namespaces::MAIN);
+        $objWriter->writeAttribute('xmlns:r', Namespaces::SCHEMA_OFFICE_DOCUMENT);
 
-        $objWriter->writeAttribute('xmlns:xdr', 'http://schemas.openxmlformats.org/drawingml/2006/spreadsheetDrawing');
-        $objWriter->writeAttribute('xmlns:x14', 'http://schemas.microsoft.com/office/spreadsheetml/2009/9/main');
-        $objWriter->writeAttribute('xmlns:xm', 'http://schemas.microsoft.com/office/excel/2006/main');
-        $objWriter->writeAttribute('xmlns:mc', 'http://schemas.openxmlformats.org/markup-compatibility/2006');
+        $objWriter->writeAttribute('xmlns:xdr', Namespaces::SPREADSHEET_DRAWING);
+        $objWriter->writeAttribute('xmlns:x14', Namespaces::DATA_VALIDATIONS1);
+        $objWriter->writeAttribute('xmlns:xm', Namespaces::DATA_VALIDATIONS2);
+        $objWriter->writeAttribute('xmlns:mc', Namespaces::COMPATIBILITY);
         $objWriter->writeAttribute('mc:Ignorable', 'x14ac');
-        $objWriter->writeAttribute('xmlns:x14ac', 'http://schemas.microsoft.com/office/spreadsheetml/2009/9/ac');
+        $objWriter->writeAttribute('xmlns:x14ac', Namespaces::SPREADSHEETML_AC);
 
         // sheetPr
         $this->writeSheetPr($objWriter, $worksheet);