To examine the value of a single variable, use C-h v
describe-variable), which reads a variable name using the
minibuffer, with completion. It displays both the value and the
documentation of the variable. For example,
C-h v fill-column RET
displays something like this:
fill-column's value is 75 Documentation: *Column beyond which automatic line-wrapping should happen. Automatically becomes buffer-local when set in any fashion.
The star at the beginning of the documentation indicates that this variable is an option. C-h v is not restricted to options; it allows any variable name.
The most convenient way to set a specific option is with M-x set-variable. This reads the variable name with the minibuffer (with completion), and then reads a Lisp expression for the new value using the minibuffer a second time. For example,
M-x set-variable RET fill-column RET 75 RET
fill-column to 75.
You can set any variable with a Lisp expression using the function
setq. Here's how to use it to set
(setq fill-column 75)
Setting variables, like all means of customizing Emacs except where otherwise stated, affects only the current Emacs session.