XLS files are Excel workbook files created in Excel 97 or Excel 2003. The 2007, 2010 and 2013 versions of the program use the XLSX format by default to save workbooks, but can also use the XLS format.
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Why your old Excel spreadsheet is 'legacy code' (and how to fix it)
Transform legacy spreadsheets into maintainable, automated tools that scale and survive the test of time.
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