The default file extension for MS Word 2010 is .docx. Word 2010 can also save in the older .doc format for compatibility with earlier versions.

File format

  • .docx : Default format in Word 2010.
  • .doc : Older Word format used for compatibility mode.

Simple answer

If you need the one main extension name, it is .docx.

TL;DR: MS Word 2010 uses .docx by default.