When working with documents created with Microsoft Word, I see files with a .DOC extension, and others with a .DOCX extension. What’s the difference? And should I prefer one over the other when I’m creating documents?
This is an important question, and a seemingly simple question; unfortunately, it has a long and complex answer.
It may seem odd that Microsoft Word has two associated file extensions, but there’s a history here. When Microsoft first released Word, it stored the contents of the document in a file with the extension .DOC. The resulting file that you stored on disk contained information that was proprietary to Microsoft, and its exact format was private, difficult to work with, and fragile.
Microsoft did document the format, so that other applications could read and write files in Microsoft Word format, but it wasn’t easy. In addition, if a .DOC file was damaged on disk, perhaps as part of a transfer, it was ruined.
In 2007, Microsoft revised the file format. Microsoft Word files were now stored in a standard ZIP file format, the file format was simple to understand and document, and any application could easily create files that Microsoft Word could load. To discern between the old-style files and the new ones, Microsoft changed the default file extension from .DOC to .DOCX.
The main advantages of the new format are that it is far easier to repair a damaged .DOCX file because of the way it’s stored on disk, and that a given .DOCX file is generally far smaller than the corresponding .DOC file would be. This makes storing and transferring/emailing .DOCX files easier.
Which should you use? Clearly, the .DOCX format is simpler, safer, and smaller. Given the choice, use it.
By the way, it’s worth noting that Microsoft made similar changes for Excel and PowerPoint documents in 2007: .XLS files because .XLSX, and .PPT files became .PPTX files. In each case, the new file format is smaller and safer, and you should use them if possible.