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Xlsx is a new spreadsheet format that was introduced by Microsoft in Microsoft Office 2007. It replaced .Xls as the standard format of Microsoft Excel Spreadsheets. Xlsx is based on the new standard for Office files(spreadsheets, word processing documents etc) called Office Open XML, which replaced the previous binary and 2003 XML formats.

Xlsx files cannot be opened in older versions of Microsoft Office without prior conversion to Xls format.

How to convert XLSx files to XLS files?

You can convert Xlsx files to Xls files using Sheet Convert: Convert XLSx Files to XLS Online for Free