This format (or the XML components of it) has been referred to as SpreadsheetML, a name also used for the standalone XML files the earlier Excel 2003 was able to generate.
Graphical inserted elements may be stored in the form of DrawingML, embedded in the XML.
This (along with the other Office Open XML items DOCX and PPTX) was initially standardized as ECMA-376 in 2006. Three formats of this standard have been produced; the second version also corresponds to ISO/IEC 29500.
Incompatibility with earlier versions
Attempting to open XLSX files with earlier versions of Excel (pre-2007) results in garbage instead of a proper spreadsheet. A compatibility pack supposedly adds the ability to load the newer format into the older versions, but this doesn't necessarily work well in all cases. This can be a problem when people insist on e-mailing you files in the newest proprietary Microsoft formats when, most of the time, whatever they're sending could have been done fine in an entirely nonproprietary keep-it-simple-stupid format such as CSV or plain text. Open Office can open XLSX files, however.
This form of XML-based spreadsheet is also different from the (also XML-based) SpreadsheetML available as a save option in earlier Excel versions, though this sort of file can still be read and written by current Excel versions.
Like the other "Open XML" formats, this file format actually consists of various files (mostly XML) compressed into a ZIP archive, with this fact obscured from the end user by the use of a different file extension.
Links and references
- ECMA-376 specification
- ISO/IEC 29500 specification
- How to open new file formats in earlier versions of Microsoft Office
- Implementing a Turing machine in Excel
- OOXML Format Family -- ISO/IEC 29500 and ECMA 376 (Library of Congress)
- XLSX Transitional (Office Open XML), Library of Congress
- XLSX Strict (Office Open XML), Library of Congress
- Markup Compatibility and Extensibility (Office Open XML), Library of Congress