difference between doc and docx

1.DOC is the default extension of Word 2003 and older while DOCX is the
default extension of Word 2007 and newer
2.Word 2003 and older cannot open DOCX files without the compatibility pack
3.DOCX is XML based while DOC is in a binary format
4.DOC is proprietary while DOCX is an open standard
5.DOCX can work with newer features while DOC cannot

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DOC is a document format used by Microsoft Word, while DOCX is its successor.
Both are relatively open, but DOCX is more efficient and creates smaller,
less corruptable files . If given the choice, use DOCX. DOC is only necessary
if the file will be used by pre-2007 versions of Word.