C-h f function RET (describe-function) 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 example,
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
function make-vector.
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.
C-h o (describe-symbol) is like
C-h f and C-h v, but it describes any
symbol, be it a function, a variable, or a face. If the symbol
has more than one definition, like it has both definition as a
function and as a variable, this command will show the
documentation of all of them, one after the other.