Microsoft Excel 2010 supports the use of an XML-based file format called the Microsoft Office Open XML format. This file format carries the filename extension .xlsx for Excel workbooks and .xlsm for macro-enabled workbooks. The XML-based file format — introduced in Excel 2007 — is more efficient, resulting in smaller file sizes and offering superior integration with external data sources. Excel 2010 automatically saves any new workbook you create with the .xlsx extension unless you choose to save the file in a different format.

Fortunately, Excel 2010 has no trouble opening any workbook files saved in the .xls file format used by Excel versions 97 through 2003. More importantly, the program automatically saves all editing changes you make to these files in this original file format, and warns you if you add a new Excel 2010 or 2007 element to the existing workbook that’s not supported by earlier versions.

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