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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.
A Brief History of the DOC Format
Microsoft Word started using the DOC format and file extension over 30 years ago in the very first release of Word for MS-DOS. As an extension explicitly for Microsoft's proprietary document processor, the format was also proprietary: Word was the only program that officially supported DOC files until Microsoft opened the specification in 2006, after which it was reverse-engineered.
In the 90s and early 2000s, various competing products could work with DOC files, though some of Word's more exotic formatting and options weren't fully supported in other word processors. Since Office and Word were the de facto standards for office productivity suites and word processors, respectively, the closed nature of the file format undoubtedly helped Microsoft retain its domination over products like Corel's WordPerfect. Since 2008, Microsoft has released and updated the DOC format specification several times for use in other programs, though not all of Word's advanced functions are supported by the open documentation.
After 2008, the DOC format was integrated into paid and free word processing programs from many vendors. It made working with older word processor formats considerably easier, and many users still prefer to save in the older DOC standard, on the off chance that a friend or client with an older version of Microsoft Office might need to open it.
The Introduction of Office Open XML (DOCX)
Under pressure from the rising competition of the free and open-source Open Office and its competing Open Document Format (ODF), Microsoft pushed for the adoption of an even broader open standard in the early 2000s. This culminated in the development of the DOCX file format, along with its companions like XLSX for spreadsheets and PPTX for presentations.
The standards were presented under the name "Office Open XML" (no relation to the Open Office program) since the formats were based on Extensible Markup Language rather than the older and less efficient binary-based format. This language allowed for a few benefits, most notably smaller file sizes, less chance of corruption, and better looking compressed images.
The XML-based DOCX format became the default save file for Word in the 2007 version of the software. At the time, many users assumed that the new DOCX format and its Microsoft Office contemporaries were merely a means for Microsoft to phase out older versions of the software and sell new copies, since older releases of Word and Office couldn't read the new XML files. This wasn't entirely true; Word 2003 can read special Word XML file formats, and compatibility updates were later applied to other versions. But in any case, some users manually saved files in the older DOC standard instead of DOCX for the sake of compatibility...somewhat ironically, since it was only more compatible with older versions of Word, not with other cross-platform tools like Open Office Writer.
Ten years later, DOCX has become the new de facto standard, though it's not quite as universal as the older DOC file format was thanks to competitors like ODF and a general decrease in traditional word processor usage.
Which One Should You Use?
DOCX is a better choice for just about every situation. The format creates smaller, lighter files that are easier to read and transfer. The open nature of the Office Open XML standard means that it can be read by just about any full-featured word processor, including online tools like Google Docs. The only reason to use the older DOC file format now would be to recover some files older than ten years, or to work with a very much out-of-date word processor. In either case, it would be best to re-save the file in DOCX, or some other modern standard like ODF, for an easy conversion.
Image Credit: WinWorld
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