Although the article at
http://word.mvps.org/FAQs/Customization/CustomizeNormalTemplate.htm explains
how to do it, it also explains why it isn't a good idea. The Normal.dot
template that supplies the settings for the "new blank document" is also
used for other kinds of documents such as labels, and making changes like
line spacing will play havoc with those documents. Instead, create a new
template for the specific type of document you want this for, and select
that template from the File > New dialog when you want a new document. See
http://word.mvps.org/FAQs/Customization/CreateATemplatePart1.htm for more
info.
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Regards,
Jay Freedman
Microsoft Word MVP FAQ: http://word.mvps.org