Files with the .xlsx extension are Microsoft Excel files that have been created using Microsoft Excel versions 2007 and later. Previous versions of Excel that were created prior to 2007 saved these spreadsheet documents in .xls format. The 2007 and later versions of Excel use the Office Open XML Format, creating the .xlsx extension.
The new Microsoft Excel technology combines XML formatting with document compression features, making the files more manageable to the end user. Up until the introduction of the .xlsx format, .xls was the most common extension used for document spreadsheets. As the later versions of Excel become more widely used, the .xlsx format will increasingly take its place.
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