If you want to customize a MajorMode, you may have to execute
some code whenever the major mode is activated.  This is done using
the major mode hook.  It usually has the same name as the major mode
with "-hook" appended.  Thus, in order to change some things in
<code>text-mode</code>, you'd add code to <code>text-mode-hook</code>.

Remember, most customizations can be set and saved using <code>M-x
customize-group</code>.  Only rarely will you need to put code on the
hook.  One such thing might be customization of font-lock keywords.
See ShowWhiteSpace for an example.

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CategoryModes