Hi dugy,

Word 2010 works with both Word 97-2003 documents (.doc extension) and Word 2007+ documents (.docx and .docm extensions). When you open an existing document, Word will automatically save it in the same format in which it was created. New documents, however, are ordinarily saved in the default format dictated by the settings under Word Options|Save > Save files in this format. The choice of a default save format should take account of whether you need the additional functions provided only by Word 2007 & later or, on the other hand, whether the file will be shared with users still using Word 97-2003.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]