GNU Emacs Manual - Locals

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### Local Variables

`M-x make-local-variable `RET` var `RET``
  Make variable var have a local value in the current buffer.
`M-x kill-local-variable `RET` var `RET``
  Make variable var use its global value in the current buffer.
`M-x make-variable-buffer-local `RET` var `RET``
  Mark variable var so that setting it will make it local to the buffer that is current at that time.

Almost any variable can be made local to a specific Emacs buffer. This means that its value in that buffer is independent of its value in other buffers. A few variables are always local in every buffer. Every other Emacs variable has a global value which is in effect in all buffers that have not made the variable local.

`M-x make-local-variable` reads the name of a variable and makes it local to the current buffer. Further changes in this buffer will not affect others, and further changes in the global value will not affect this buffer.

`M-x make-variable-buffer-local` reads the name of a variable and changes the future behavior of the variable so that it will become local automatically when it is set. More precisely, once a variable has been marked in this way, the usual ways of setting the variable automatically do `make-local-variable` first. We call such variables per-buffer variables.

Major modes (see section Major Modes) always make variables local to the buffer before setting the variables. This is why changing major modes in one buffer has no effect on other buffers. Minor modes also work by setting variables--normally, each minor mode has one controlling variable which is non-`nil` when the mode is enabled (see section Minor Modes). For most minor modes, the controlling variable is per buffer.

Emacs contains a number of variables that are always per-buffer. These include `abbrev-mode`, `auto-fill-function`, `case-fold-search`, `comment-column`, `ctl-arrow`, `fill-column`, `fill-prefix`, `indent-tabs-mode`, `left-margin`, `mode-line-format`, `overwrite-mode`, `selective-display-ellipses`, `selective-display`, `tab-width`, and `truncate-lines`. Some other variables are always local in every buffer, but they are used for internal purposes.

A few variables cannot be local to a buffer because they are always local to each display instead (see section Multiple Displays). If you try to make one of these variables buffer-local, you'll get an error message.

`M-x kill-local-variable` reads the name of a variable and makes it cease to be local to the current buffer. The global value of the variable henceforth is in effect in this buffer. Setting the major mode kills all the local variables of the buffer except for a few variables specially marked as permanent locals.

To set the global value of a variable, regardless of whether the variable has a local value in the current buffer, you can use the Lisp construct `setq-default`. This construct is used just like `setq`, but it sets variables' global values instead of their local values (if any). When the current buffer does have a local value, the new global value may not be visible until you switch to another buffer. Here is an example:

    (setq-default fill-column 75)

`setq-default` is the only way to set the global value of a variable that has been marked with `make-variable-buffer-local`.

Lisp programs can use `default-value` to look at a variable's default value. This function takes a symbol as argument and returns its default value. The argument is evaluated; usually you must quote it explicitly. For example, here's how to obtain the default value of `fill-column`:

    (default-value 'fill-column)

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