File Format

A key consideration is file compatibility. Excel 97 through Excel 2003 all use the same file format, so file compatibility is not a problem for these four versions. Excel 2007, however, uses a new file format. Fortunately, Microsoft has made a "compatibility pack" available for earlier versions of Excel. This compatibility pack enables older versions of Excel to read and write the new XLSX file format.

It's important to understand the difference between file compatibility and feature compatibility. For example, even though the compatibility pack enables Excel 2003 to open files created by Excel 2007, it cannot handle features that were introduced in later versions.

CROSS-REFERENCE  

Refer to Chapter 4 for more information about Excel's file format, and read Chapter 26 for more information about compatibility issues for developers.