efaq: Changing the length of a Tab

 
 5.32 How do I change Emacs’s idea of the <TAB> character’s length?
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 Set the default value of the variable ‘tab-width’.  For example, to set
 <TAB> stops every 10 characters, insert the following in your ‘.emacs’
 file:
 
      (setq-default tab-width 10)
 
    Do not confuse variable ‘tab-width’ with variable ‘tab-stop-list’.
 The former is used for the display of literal <TAB> characters.  The
 latter controls what characters are inserted when you press the <TAB>
 character in certain modes.