C-h f function <RET> (
displays the documentation of Lisp function function, in a
window. Since commands are Lisp functions, you can use this method to
view the documentation of any command whose name you know. For
C-h f auto-fill-mode <RET>
displays the documentation of
auto-fill-mode. This is the only
way to get the documentation of a command that is not bound to any key
(one which you would normally run using M-x).
C-h f is also useful for Lisp functions that you use in a Lisp
program. For example, if you have just written the expression
(make-vector len) and want to check that you are using
make-vector properly, type C-h f make-vector <RET>.
Because C-h f allows all function names, not just command names,
you may find that some of your favorite completion abbreviations that
work in M-x don't work in C-h f. An abbreviation that is
unique among command names may not be unique among all function names.
If you type C-h f <RET>, it describes the function called
by the innermost Lisp expression in the buffer around point,
provided that function name is a valid, defined Lisp function.
(That name appears as the default while you enter the argument.) For
example, if point is located following the text ‘(make-vector
(car x)’, the innermost list containing point is the one that starts
with ‘(make-vector’, so C-h f <RET> describes the
C-h f is also useful just to verify that you spelled a function name correctly. If the minibuffer prompt for C-h f shows the function name from the buffer as the default, it means that name is defined as a Lisp function. Type C-g to cancel the C-h f command if you don't really want to view the documentation.
C-h v (
describe-variable) is like C-h f but
describes Lisp variables instead of Lisp functions. Its default is
the Lisp symbol around or before point, if that is the name of a
defined Lisp variable. See Variables.
Help buffers that describe Emacs variables and functions normally have hyperlinks to the corresponding source code, if you have the source files installed (see Hyperlinking).
To find a command's documentation in a manual, use C-h F
Info-goto-emacs-command-node). This knows about various
manuals, not just the Emacs manual, and finds the right one.