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.