"Master Document" is a term of art in Word referring to a "feature" that not
only doesn't work but also destroys documents. The consensus (with the
limited exception of Steve Hudson) among those offering advice on these
newsgroups is that using the Master Document feature is a sure way to
destroy your document. It can destroy parts of your document that you are
not even working on! I think John McGhie said it succinctly when he said
that there are two kinds of Master Documents: Those that are corrupt and
those that will be corrupt soon. See  <URL:
http://www.addbalance.com/word/masterdocuments.htm> for information on the
Master Document feature and workarounds. (This page also has a link to Steve
Hudson's chapter on how he gets Master Documents to work.) See <URL:
http://www.mvps.org/word/FAQs/General/WhyMasterDocsCorrupt.htm> for more
information on what goes wrong, and <URL:
http://www.mvps.org/word/FAQs/General/RecoverMasterDocs.htm> for ideas on
how to salvage what you can.