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.

Image: Microsoft The demonstration .xlsx file contains only one sheet because that’s all the .csv feature supports. Excel won’t allow you to save a multiple-sheet workbook to .csv format.