setq-default

setq-default is a special form in `C source code'.

(setq-default [VAR VALUE]...)

Set the default value of variable VAR to VALUE.
VAR, the variable name, is literal (not evaluated);
VALUE is an expression: it is evaluated and its value returned.
The default value of a variable is seen in buffers
that do not have their own values for the variable.

More generally, you can use multiple variables and values, as in
(setq-default VAR VALUE VAR VALUE...)
This sets each VAR's default value to the corresponding VALUE.
The VALUE for the Nth VAR can refer to the new default values
of previous VARs.