Some other variables are always local in every buffer, but they are used for internal purposes. 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 specially marked variables that are 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 function setq-default . It works like setq .
Lisp programs can look at a variable's default value with default-value .
This function takes a symbol as argument and returns its default value.