(vhdl-reset-active-high, vhdl-clock-rising-edge): Doc fix (Nil -> nil).

[gnu-emacs] / man / help.texi

@c This is part of the Emacs manual.
@c Copyright (C) 1985, 1986, 1987, 1993, 1994, 1995, 1997, 2000, 2001,
@c   2002, 2003, 2004, 2005, 2006 Free Software Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Help, Mark, M-x, Top
@chapter Help
@kindex Help
@cindex help
@cindex self-documentation
@findex help-command
@kindex C-h
@kindex F1

  Emacs provides extensive help features, all accessible through the
@dfn{help character}, @kbd{C-h}.  This is a prefix key that is used
for commands that display documentation; the next character you type
should be a @dfn{help options}, to ask for a particular kind of help.
You can cancel the @kbd{C-h} command with @kbd{C-g}.  The function key
@key{F1} is equivalent to @kbd{C-h}.

@kindex C-h C-h
@findex help-for-help
  @kbd{C-h} itself is one of the help options; @kbd{C-h C-h} displays
a list of help options, with a brief description of each one
(@code{help-for-help}).  You can scroll the list with @key{SPC} and
@key{DEL}, then type the help option you want.  To cancel, type
@kbd{C-g}.

  @kbd{C-h} or @key{F1} means ``help'' in various other contexts as
well.  For instance, you can type them after a prefix key to display
list of the keys that can follow the prefix key.  (A few prefix keys
don't support @kbd{C-h} in this way, because they define other
meanings for it, but they all support @key{F1} for help.)

  Most help buffers use a special major mode, Help mode, which lets
you scroll conveniently with @key{SPC} and @key{DEL}.  You can also
follow hyperlinks to URLs, and to other facilities including Info
nodes and customization buffers.  @xref{Help Mode}.

@cindex searching documentation efficiently
@cindex looking for a subject in documentation
  If you are looking for a certain feature, but don't know what it is
called or where to look, we recommend three methods.  First, try an
apropos command, then try searching the manual index, then look in the
FAQ and the package keywords.

@table @kbd
@item C-h a @var{topics} @key{RET}
This searches for commands whose names match the argument
@var{topics}.  The argument can be a keyword, a list of keywords, or a
regular expression (@pxref{Regexps}).  This command displays all the
matches in a new buffer.  @xref{Apropos}.

@item C-h i d m emacs @key{RET} i @var{topic} @key{RET}
This searches for @var{topic} in the indices of the on-line Emacs
manual, and displays the first match found.  Press @kbd{,} to see
subsequent matches.  You can use a regular expression as @var{topic}.

@item C-h i d m emacs @key{RET} s @var{topic} @key{RET}
Similar, but searches the @emph{text} of the manual rather than the
indices.

@item C-h C-f
This displays the Emacs FAQ.  You can use the Info commands
to browse it.

@item C-h p
This displays the available Emacs packages based on keywords.
@xref{Library Keywords}.
@end table

@menu
* Help Summary::        Brief list of all Help commands.
* Key Help::            Asking what a key does in Emacs.
* Name Help::           Asking about a command, variable or function name.
* Apropos::             Asking what pertains to a given topic.
* Help Mode::           Special features of Help mode and Help buffers.
* Library Keywords::    Finding Lisp libraries by keywords (topics).
* Language Help::       Help relating to international language support.
* Misc Help::           Other help commands.
* Help Files::          Commands to display pre-written help files.
* Help Echo::           Help on active text and tooltips (`balloon help')
@end menu

@iftex
@node Help Summary
@end iftex
@ifnottex
@node Help Summary
@section Help Summary
@end ifnottex

  Here is a summary of the Emacs interactive help commands.  (The
character that follows @kbd{C-h} is the ``help option.'')  @xref{Help
Files}, for other help commands that display fixed files of
information.

@table @kbd
@item C-h a @var{topics} @key{RET}
Display a list of commands whose names match @var{topics}
(@code{apropos-command}; @pxref{Apropos}).
@item C-h b
Display all active key bindings; minor mode bindings first, then those
of the major mode, then global bindings (@code{describe-bindings}).
@item C-h c @var{key}
Given a key sequence @var{key}, show the name of the command that it
runs (@code{describe-key-briefly}).  Here @kbd{c} stands for
``character.''  For more extensive information on @var{key}, use
@kbd{C-h k}.
@item C-h d @var{topics} @key{RET}
Display the commands and variables whose documentation matches
@var{topics} (@code{apropos-documentation}).
@item C-h e
Display the @code{*Messages*} buffer
(@code{view-echo-area-messages}).
@item C-h f @var{function} @key{RET}
Display documentation on the Lisp function named @var{function}
(@code{describe-function}).  Since commands are Lisp functions,
this works for commands too.
@item C-h h
Display the @file{HELLO} file, which shows examples of various character
sets.
@item C-h i
Run Info, the GNU documentation browser (@code{info}).
The complete Emacs manual is available on-line in Info.
@item C-h k @var{key}
Display the name and documentation of the command that @var{key} runs
(@code{describe-key}).
@item C-h l
Display a description of the last 100 characters you typed
(@code{view-lossage}).
@item C-h m
Display documentation of the current major mode (@code{describe-mode}).
@item C-h p
Find packages by topic keyword (@code{finder-by-keyword}).
@item C-h s
Display the current contents of the syntax table, with an explanation of
what they mean (@code{describe-syntax}).  @xref{Syntax}.
@item C-h t
Enter the Emacs interactive tutorial (@code{help-with-tutorial}).
@item C-h v @var{var} @key{RET}
Display the documentation of the Lisp variable @var{var}
(@code{describe-variable}).
@item C-h w @var{command} @key{RET}
Show which keys run the command named @var{command} (@code{where-is}).
@item C-h C @var{coding} @key{RET}
Describe the coding system @var{coding}
(@code{describe-coding-system}).
@item C-h C @key{RET}
Describe the coding systems currently in use.
@item C-h I @var{method} @key{RET}
Describe the input method @var{method} (@code{describe-input-method}).
@item C-h L @var{language-env} @key{RET}
Display information on the character sets, coding systems, and input
methods used in language environment @var{language-env}
(@code{describe-language-environment}).
@item C-h F @var{function} @key{RET}
Enter Info and goes to the node that documents the Emacs function
@var{function} (@code{Info-goto-emacs-command-node}).
@item C-h K @var{key}
Enter Info and goes to the node that documents the key sequence
@var{key} (@code{Info-goto-emacs-key-command-node}).
@item C-h S @var{symbol} @key{RET}
Display the Info documentation on symbol @var{symbol} according to the
programming language you are editing (@code{info-lookup-symbol}).
@item C-h .
Display the help message for a special text area, if point is in one
(@code{display-local-help}).  (These include, for example, links in
@samp{*Help*} buffers.)
@end table

@node Key Help
@section Documentation for a Key

@kindex C-h c
@findex describe-key-briefly
  The help commands to get information about a key sequence are
@kbd{C-h c} and @w{@kbd{C-h k}}.  @kbd{C-h c @var{key}} displays in
the echo area the name of the command that @var{key} is bound to.  For
example, @kbd{C-h c C-f} displays @samp{forward-char}.  Since command
names are chosen to describe what the commands do, this gives you a
very brief description of what @var{key} does.

@kindex C-h k
@findex describe-key
  @kbd{C-h k @var{key}} is similar but gives more information: it
displays the documentation string of the command as well as its name.
It displays this information in a window, since it may not fit in the
echo area.

@kindex C-h K
@findex Info-goto-emacs-key-command-node
  To find the documentation of a key sequence @var{key}, type @kbd{C-h
K @var{key}}.  This displays the appropriate manual section which
contains the documentation of @var{key}.

  @kbd{C-h c}, @kbd{C-h k} and @kbd{C-h K} work for any sort of key
sequences, including function keys, menus, and mouse events.  For
instance, after @kbd{C-h k} you can select a menu item from the menu
bar, to view the documentation string of the command it runs.

@kindex C-h w
@findex where-is
  @kbd{C-h w @var{command} @key{RET}} lists the keys that are bound to
@var{command}.  It displays the list in the echo area.  If it says the
command is not on any key, that means you must use @kbd{M-x} to run
it.  @kbd{C-h w} runs the command @code{where-is}.

@node Name Help
@section Help by Command or Variable Name

@kindex C-h f
@findex describe-function
  @kbd{C-h f @var{function} @key{RET}} (@code{describe-function})
displays the documentation of Lisp function @var{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,

@example
C-h f auto-fill-mode @key{RET}
@end example

@noindent
displays the documentation of @code{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 @kbd{M-x}).

  @kbd{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
@code{(make-vector len)} and want to check that you are using
@code{make-vector} properly, type @kbd{C-h f make-vector @key{RET}}.
Because @kbd{C-h f} allows all function names, not just command names,
you may find that some of your favorite completion abbreviations that
work in @kbd{M-x} don't work in @kbd{C-h f}.  An abbreviation that is
unique among command names may not be unique among all function names.

  If you type @kbd{C-h f @key{RET}}, it describes the function called
by the innermost Lisp expression in the buffer around point,
@emph{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 @samp{(make-vector
(car x)}, the innermost list containing point is the one that starts
with @samp{(make-vector}, so @kbd{C-h f @key{RET}} will describe the
function @code{make-vector}.

  @kbd{C-h f} is also useful just to verify that you spelled a
function name correctly.  If the minibuffer prompt for @kbd{C-h f}
shows the function name from the buffer as the default, it means that
name is defined as a Lisp function.  Type @kbd{C-g} to cancel the
@kbd{C-h f} command if you don't really want to view the
documentation.

@kindex C-h v
@findex describe-variable
  @kbd{C-h v} (@code{describe-variable}) is like @kbd{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.  @xref{Variables}.

  Help buffers that describe Emacs variables and functions normally
have hyperlinks to the corresponding source definition, if you have
the source files installed.  (@xref{Hyperlinking}.)  If you know Lisp
(or C), this provides the ultimate documentation.  If you don't know
Lisp, you should learn it.  (The Introduction to Emacs Lisp
Programming, available from the FSF through fsf.org, is a good way to
get started.)  If Emacs feels you are just @emph{using} it, treating
it as an object program, its feelings may be hurt.  For real intimacy,
read the Emacs source code.

@kindex C-h F
@findex Info-goto-emacs-command-node
  To find a function's documentation in a manual, use @kbd{C-h F}
(@code{Info-goto-emacs-command-node}).  This knows about various
manuals, not just the Emacs manual, and finds the right one.

@node Apropos
@section Apropos

  The @dfn{apropos} commands answer questions like, ``What are the
commands for working with files?''  More precisely, you specify an
@dfn{apropos pattern}, which means either a word, a list of words, or
a regular expression.  Each apropos command displays a list of items
that match the pattern, in a separate buffer.

@table @kbd
@item C-h a @var{pattern} @key{RET}
Search for commands whose names match @var{pattern}.

@item M-x apropos @key{RET} @var{pattern} @key{RET}
Search for functions and variables whose names match @var{pattern}.
Both interactive functions (commands) and noninteractive functions can
be found by this command.

@item M-x apropos-variable @key{RET} @var{pattern} @key{RET}
Search for user-option variables whose names match @var{pattern}.

@item M-x apropos-value @key{RET} @var{pattern} @key{RET}
Search for functions whose definitions @var{pattern}, and variables
whose values match @var{pattern}.

@item C-h d @var{pattern} @key{RET}
Search for functions and variables whose @strong{documentation
strings} match @var{pattern}.
@end table

@kindex C-h a
@findex apropos-command
@cindex apropos
  The simplest kind of apropos pattern is one word.  Anything which
contains that word matches the pattern.  Thus, to find the commands
that work on files, type @kbd{C-h a file @key{RET}}.  This displays a
list of all command names that contain @samp{file}, including
@code{copy-file}, @code{find-file}, and so on.  Each command name
comes with a brief description and a list of keys you can currently
invoke it with.  In our example, it would say that you can invoke
@code{find-file} by typing @kbd{C-x C-f}.

  The @kbd{a} in @kbd{C-h a} stands for ``Apropos''; @kbd{C-h a}
runs the command @code{apropos-command}.  This command normally checks
only commands (interactive functions); if you specify a prefix
argument, it checks noninteractive functions as well.

  For more information about a function definition, variable or symbol
property listed in the apropos buffer, you can click on it with
@kbd{Mouse-1} or @kbd{Mouse-2}, or move there and type @key{RET}.

  When you specify more than one word in the apropos pattern, a name
must contain at least two of the words in order to match.  Thus, if
you are looking for commands to kill a chunk of text before point, you
could try @kbd{C-h a kill back backward behind before @key{RET}}.  The
real command name @code{kill-backward} will match that; if there were
a command @code{kill-text-before}, it would also match, since it
contains two of the specified words.

  For even greater flexibility, you can specify a regular expression
(@pxref{Regexps}).  An apropos pattern is interpreted as a regular
expression if it contains any of the regular expression special
characters, @samp{^$*+?.\[}.

  Following the conventions for naming Emacs commands, here are some
words that you'll find useful in apropos patterns.  By using them in
@kbd{C-h a}, you will also get a feel for the naming conventions.

@quotation
char, line, word, sentence, paragraph, region, page, sexp, list, defun,
rect, buffer, frame, window, face, file, dir, register, mode, beginning, end,
forward, backward, next, previous, up, down, search, goto, kill, delete,
mark, insert, yank, fill, indent, case, change, set, what, list, find,
view, describe, default.
@end quotation

@findex apropos
  Use @kbd{M-x apropos} instead of @kbd{C-h a} to list all the Lisp
symbols that match an apropos pattern, not just the symbols that are
commands.  This command does not list key bindings by default; specify
a numeric argument if you want it to list them.

@findex apropos-variable
  Use @kbd{M-x apropos-variable} to list user-customizable variables
that match an apropos pattern.  If you specify a prefix argument, it
lists all matching variables.

@kindex C-h d
@findex apropos-documentation
  The @code{apropos-documentation} command is like @code{apropos}
except that it searches documentation strings instead of symbol names
for matches.

@findex apropos-value
  The @code{apropos-value} command is like @code{apropos} except that
it searches variables' values for matches for the apropos pattern.
With a prefix argument, it also checks symbols' function definitions
and property lists.

@vindex apropos-do-all
  If the variable @code{apropos-do-all} is non-@code{nil}, the apropos
commands always behave as if they had been given a prefix argument.

@vindex apropos-sort-by-scores
@cindex apropos search results, order by score
  By default, apropos lists the search results in alphabetical order.
If the variable @code{apropos-sort-by-scores} is non-@code{nil}, the
apropos commands try to guess the relevance of each result, and
display the most relevant ones first.

@vindex apropos-documentation-sort-by-scores
  By default, apropos lists the search results for
@code{apropos-documentation} in order of relevance of the match.  If
the variable @code{apropos-documentation-sort-by-scores} is
@code{nil}, apropos lists the symbols found in alphabetical order.

@node Help Mode
@section Help Mode Commands

  Help buffers provide the same commands as View mode (@pxref{Misc File
Ops}), plus a few special commands of their own.

@table @kbd
@item @key{SPC}
Scroll forward.
@item @key{DEL}
Scroll backward.
@item @key{RET}
Follow a cross reference at point.
@item @key{TAB}
Move point forward to the next cross reference.
@item S-@key{TAB}
Move point back to the previous cross reference.
@item Mouse-1
@itemx Mouse-2
Follow a cross reference that you click on.
@item C-c C-c
Show all documentation about the symbol at point.
@end table

  When a function name (@pxref{M-x,, Running Commands by Name}),
variable name (@pxref{Variables}), or face name (@pxref{Faces})
appears in the documentation, it normally appears inside paired
single-quotes.  To view the documentation of that command, variable or
face, you can click on the name with @kbd{Mouse-1} or @kbd{Mouse-2},
or move point there and type @key{RET}.  Use @kbd{C-c C-b} to retrace
your steps.

@cindex URL, viewing in help
@cindex help, viewing web pages
@cindex viewing web pages in help
@cindex web pages, viewing in help
@findex browse-url
  You can follow cross references to URLs (web pages) also.  This uses
the @code{browse-url} command to view the page in the browser you
choose.  @xref{Browse-URL}.

@kindex @key{TAB} @r{(Help mode)}
@findex help-next-ref
@kindex S-@key{TAB} @r{(Help mode)}
@findex help-previous-ref
  There are convenient commands to move point to cross references in
the help text.  @key{TAB} (@code{help-next-ref}) moves point down to
the next cross reference.  @kbd{S-@key{TAB}} moves up to the previous
cross reference (@code{help-previous-ref}).

  To view all documentation about any symbol name that appears in the
text, move point to the symbol name and type @kbd{C-c C-c}
(@code{help-follow-symbol}).  This shows all available documentation
about the symbol as a variable, function and/or face.  As above, use
@kbd{C-c C-b} to retrace your steps.

@node Library Keywords
@section Keyword Search for Lisp Libraries

@kindex C-h p
@findex finder-by-keyword
The @kbd{C-h p} command lets you search the standard Emacs Lisp
libraries by topic keywords.  Here is a partial list of keywords you can
use:

@multitable {convenience} {aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa}
@item abbrev@tab abbreviation handling, typing shortcuts, macros.
@item bib@tab code related to the @code{bib} bibliography processor.
@item c@tab support for the C language and related languages.
@item calendar@tab calendar and time management support.
@item comm@tab communications, networking, remote access to files.
@item convenience@tab convenience features for faster editing.
@item data@tab support for editing files of data.
@item docs@tab support for Emacs documentation.
@item emulations@tab emulations of other editors.
@item extensions@tab Emacs Lisp language extensions.
@item faces@tab support for multiple fonts.
@item files@tab support for editing and manipulating files.
@item frames@tab support for Emacs frames and window systems.
@item games@tab games, jokes and amusements.
@item hardware@tab support for interfacing with exotic hardware.
@item help@tab support for on-line help systems.
@item hypermedia@tab support for links between text or other media types.
@item i18n@tab internationalization and alternate character-set support.
@item internal@tab code for Emacs internals, build process, defaults.
@item languages@tab specialized modes for editing programming languages.
@item lisp@tab Lisp support, including Emacs Lisp.
@item local@tab code local to your site.
@item maint@tab maintenance aids for the Emacs development group.
@item mail@tab modes for electronic-mail handling.
@item matching@tab various sorts of searching and matching.
@item mouse@tab mouse support.
@item multimedia@tab images and sound support.
@item news@tab support for netnews reading and posting.
@item oop@tab support for object-oriented programming.
@item outlines@tab support for hierarchical outlining.
@item processes@tab process, subshell, compilation, and job control support.
@item terminals@tab support for terminal types.
@item tex@tab supporting code for the @TeX{} formatter.
@item tools@tab programming tools.
@item unix@tab front-ends/assistants for, or emulators of, UNIX-like features.
@item wp@tab word processing.
@end multitable

@node Language Help
@section Help for International Language Support

  You can use the command @kbd{C-h L}
(@code{describe-language-environment}) to get information about a
specific language environment.  @xref{Language Environments}.  This
tells you which languages this language environment supports.  It also
lists the character sets, coding systems, and input methods that work
with this language environment, and finally shows some sample text to
illustrate scripts.

  The command @kbd{C-h h} (@code{view-hello-file}) displays the file
@file{etc/HELLO}, which shows how to say ``hello'' in many languages.

  The command @kbd{C-h I} (@code{describe-input-method}) describes an
input method---either a specified input method, or by default the
input method currently in use.  @xref{Input Methods}.

  The command @kbd{C-h C} (@code{describe-coding-system}) describes
coding systems---either a specified coding system, or the ones
currently in use.  @xref{Coding Systems}.

@node Misc Help
@section Other Help Commands

@kindex C-h i
@findex info
@cindex Info
@cindex manuals, on-line
@cindex on-line manuals
  @kbd{C-h i} (@code{info}) runs the Info program, which browses
structured documentation files.  The entire Emacs manual is available
within Info, along with many other manuals for the GNU system.  Type
@kbd{h} after entering Info to run a tutorial on using Info.

@cindex find Info manual by its file name
  With a numeric argument @var{n}, @kbd{C-h i} selects the Info buffer
@samp{*info*<@var{n}>}.  This is useful if you want to browse multiple
Info manuals simultaneously.  If you specify just @kbd{C-u} as the
prefix argument, @kbd{C-h i} prompts for the name of a documentation
file, so you can browse a file which doesn't have an entry in the
top-level Info menu.

  The help commands @kbd{C-h F @var{function} @key{RET}} and @kbd{C-h
K @var{key}}, described above, enter Info and go straight to the
documentation of @var{function} or @var{key}.

@kindex C-h S
@findex info-lookup-symbol
  When editing a program, if you have an Info version of the manual
for the programming language, you can use @kbd{C-h S}
(@code{info-lookup-symbol}) to find symbol (keyword, function or
variable) in the proper manual.  The details of how this command works
depend on the major mode.

@kindex C-h l
@findex view-lossage
  If something surprising happens, and you are not sure what you
typed, use @kbd{C-h l} (@code{view-lossage}).  @kbd{C-h l} displays
the last 100 characters you typed in Emacs.  If you see commands that
you don't know, you can use @kbd{C-h c} to find out what they do.

@kindex C-h e
@findex view-echo-area-messages
  To review recent echo area messages, use @kbd{C-h e}
(@code{view-echo-area-messages}).  This displays the buffer
@code{*Messages*}, where those messages are kept.

@kindex C-h m
@findex describe-mode
  Each Emacs major mode typically redefines a few keys and makes other
changes in how editing works.  @kbd{C-h m} (@code{describe-mode})
displays documentation on the current major mode, which normally
describes the commands and features that are changed in this mode.

@kindex C-h b
@findex describe-bindings
  @kbd{C-h b} (@code{describe-bindings}) and @kbd{C-h s}
(@code{describe-syntax}) show other information about the current
environment within Emacs.  @kbd{C-h b} displays a list of all the key
bindings now in effect: first the local bindings of the current minor
modes, then the local bindings defined by the current major mode, and
finally the global bindings (@pxref{Key Bindings}).  @kbd{C-h s}
displays the contents of the syntax table, with explanations of each
character's syntax (@pxref{Syntax}).

  You can get a list of subcommands for a particular prefix key by
typing @kbd{C-h} after the prefix key.  (There are a few prefix keys
for which this does not work---those that provide their own bindings
for @kbd{C-h}.  One of these is @key{ESC}, because @kbd{@key{ESC} C-h}
is actually @kbd{C-M-h}, which marks a defun.)

@node Help Files
@section Help Files

  The Emacs help commands described above display dynamic help based
on the current state within Emacs, or refer to manuals.  Other help
commands display pre-written, static help files.  These commands all
have the form @kbd{C-h C-@var{char}}; that is, @kbd{C-h} followed by a
control character.

@kindex C-h C-c
@findex describe-copying
@kindex C-h C-d
@findex describe-distribution
@kindex C-h C-e
@findex view-emacs-problems
@kindex C-h C-f
@findex view-emacs-FAQ
@kindex C-h C-n
@findex view-emacs-news
@kindex C-h C-p
@findex describe-project
@kindex C-h C-t
@findex view-emacs-todo
@kindex C-h C-w
@findex describe-no-warranty

@table @kbd
@item C-h C-c
Display the Emacs copying conditions (@code{describe-copying}).
These are the rules under which you can copy and redistribute Emacs.
@item C-h C-d
Display how to download or order the latest version of
Emacs and other GNU software (@code{describe-distribution}).
@item C-h C-e
Display the list of known Emacs problems, sometimes with suggested
workarounds (@code{view-emacs-problems}).
@item C-h C-f
Display the Emacs frequently-answered-questions list (@code{view-emacs-FAQ}).
@item C-h C-n
Display the Emacs ``news'' file, which lists new features in the most
recent version of Emacs (@code{view-emacs-news}).
@item C-h C-p
Display general information about the GNU Project
(@code{describe-project}).
@item C-h C-t
Display the Emacs to-do list (@code{view-todo}).
@item C-h C-w
Display the full details on the complete absence of warranty for GNU
Emacs (@code{describe-no-warranty}).
@end table

@node Help Echo
@section Help on Active Text and Tooltips

@cindex tooltips
@cindex balloon help
  When a region of text is ``active,'' so that you can select it with
the mouse or a key like @kbd{RET}, it often has associated help text.
For instance, most parts of the mode line have help text.  On
graphical displays, the help text is displayed as a ``tooltip''
(sometimes known as ``balloon help''), when you move the mouse over
the active text.  @xref{Tooltips}.  On some systems, it is shown in
the echo area.  On text-only terminals, if Emacs cannot follow the
mouse, it cannot show the help text on mouse-over.

@kindex C-h .
@findex display-local-help
@vindex help-at-pt-display-when-idle
  You can also access text region help info using the keyboard.  The
command @kbd{C-h .} (@code{display-local-help}) displays any help text
associated with the text at point, using the echo area.  If you want
help text to be displayed automatically whenever it is available at
point, set the variable @code{help-at-pt-display-when-idle} to
@code{t}.

@ignore
   arch-tag: 6f33ab62-bc75-4367-8057-fd67cc15c3a1
@end ignore