The .docx extension is assigned to Microsoft Word documents created in the 2007 edition of Microsoft Word (and all later editions). This is a new Word format that is based on XML, which allows for better compression of Word documents and reduced file sizes. The difference between the XML Word format and prior Word formats is that XML Word documents are saved in text format instead of the older binary format.
The DOCX format is known as the Microsoft Office Open XML format. Documents that are saved in .docx format are not compatible with older versions of Microsoft Word, although some open-source software applications (such as Open Office) are able to open the documents that are saved with the .docx file extension.
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