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.
Running the Compatibility Checker
You also can run the Compatibility Checker in Excel 2010 to find any potential compatibility issues if you plan to save the file in the Excel 97-2003 file format. To use this feature, follow these steps:
In Excel 2010, open the workbook you want to check.
Click the File tab, click the Check for Issues button on the Info tab (on the left), and click Check Compatibility.
The Microsoft Excel – Compatibility Checker dialog box appears. Any features that might cause issues are listed in the Summary box.
Click OK to close the dialog box.
If you’re working in an office where all the workbooks you produce with Excel 2010 must be saved in the old Excel 97-2003 file format for compatibility, you can change the program’s default Save setting so that the program always saves all new workbooks in the older file format. To do this, open the Save tab of the Excel Options dialog box (File→Options→Save) and then select Excel 97-2003 Workbook in the Save Files in This Format drop-down list box.
Displaying filename extensions within Excel
By default, filename extensions such as .xlsx and .xls do not appear as part of the filename in the Save As dialog box. However, you can change a setting in Windows to display these filename extensions. Follow these steps:
Open Windows Explorer.
You can do this quickly by opening any folder on your desktop.
Choose Tools→Folder Options.
You may need to press the Alt key in order to see the menu bar. The Folder Options dialog box appears.
Click the View tab.
Remove the check mark from the Hide Extensions for Known File Types option.
Excel 2010 also supports a binary file format called Office Excel 2010 Binary, or BIFF12, that carries the .xlsb filename extension. Select this binary format for huge spreadsheets that you create that have to be backward compatible with earlier versions of Excel.