The XLS workbook files produced by Excel 2003 use the same file format as Excel 97, Excel 2000, Excel 2002, and Excel 2003. These files cannot be opened by any version of Excel prior to Excel 97. You can, however, save a workbook using any of the older Excel file formats. You might lose some information that is specific to the later file format.
An Excel workbook or add-in file can have any extension that you like. In other words, these files don't need to be stored with an XLS (for workbooks) or XLA (for add-ins) extension.
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