Why Put "X" at the end of doc extension?

Microsoft Word 2007. Why put "X" at the end of "doc" Extension? What didn't "doc" extension do? What now "docx" extension does? Any solid reason? Please give your answer in one short line.

https://msdn.microsoft.com/en-us/library/aa338205(v=office.12).aspx

It was an (at the time) new XML-based format, and most word processors would not have been able to automatically detect and handle it if it were simply a .doc extension, so I'm assuming that's why they decided a distinct file extension was appropriate.

The main reason is that there was a huge format change from a proprietary encoding of the documents to an XML based format. They then renamed the extension with an "x" at the end to signify this. There is also the issue that the format is not backwards compatible with older Word products unless you download a patch, so this makes it that these products don't try an open the documents. It also gives an indication to other non Word products if they can read the file since it's XML based.

Here is a relevant article that goes a bit more into details :http://blog.online-convert.com/doc-vs-docx-file-extensions/