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From Information about Isaak, Brock University's Sakai-Based LMS
Microsoft Excel is a very popular spreadsheet program.
XLS is the file format used natively by Microsoft Excel 2002 and earlier.
Microsoft Excel up until 2007 version used a proprietary binary file format called Binary Interchange File Format (BIFF) as its primary format. Excel 2007 uses Office Open XML as its primary file format, an XML-based format that followed after a previous XML-based format called "XML Spreadsheet" ("XMLSS"), first introduced in Excel 2002.