For most of its long history, Microsoft Word has used a proprietary format for its saved files, DOC. Starting in 2007 with the updated version of Word (and Microsoft Office), the default save format was changed to DOCX. This wasn’t simply a belated 1990s “extreme” version of the format—that extra X stands for the Office Open XML standard. What’s the difference, and which one should you use?
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.
Comment