Hi Canna,

The docx format is a completely new format used by Word 2007 & later. It supports a range of features not supported in the earlier doc format, such as Content Controls, Citations, etc. Word 2007 & later work equally well with doc and docx files and there's no need to convert any of your old documents unless you have a need to use the new features with them. And, if you prefer, Word 2007 & later can be configured to save any new documents in the older doc format by default.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]