+@node Undo
+@section Undo
+@cindex undo
+@cindex changes, undoing
+
+ The @dfn{undo} commands undo recent changes in the buffer's text.
+Each buffer records changes individually, and the undo command always
+applies to the current buffer. You can undo all the changes in a
+buffer for as far as back these records go. Usually each editing
+command makes a separate entry in the undo records, but some commands
+such as @code{query-replace} divide their changes into multiple
+entries for flexibility in undoing. Meanwhile, self-inserting
+characters are usually grouped to make undoing less tedious.
+
+@table @kbd
+@item C-x u
+@itemx C-_
+@itemx C-/
+Undo one entry in the current buffer's undo records (@code{undo}).
+@end table
+
+@kindex C-x u
+@kindex C-_
+@kindex C-/
+@findex undo
+ To begin to undo, type the command @kbd{C-x u} (or its aliases,
+@kbd{C-_} or @kbd{C-/}). This undoes the most recent change in the
+buffer, and moves point back to where it was before that change.
+
+ Consecutive repetitions of @kbd{C-x u} (or its aliases) undo earlier
+and earlier changes in the current buffer, back to the limit of the
+current buffer's undo records. If all the recorded changes have
+already been undone, the undo command just signals an error.
+
+ If you notice that a buffer has been modified accidentally, the
+easiest way to recover is to type @kbd{C-_} repeatedly until the stars
+disappear from the front of the mode line. At this time, all the
+modifications you made have been canceled. Whenever an undo command
+makes the stars disappear from the mode line, it means that the buffer
+contents are the same as they were when the file was last read in or
+saved.
+
+ If you do not remember whether you changed the buffer deliberately,
+type @kbd{C-_} once. When you see the last change you made undone, you
+will see whether it was an intentional change. If it was an accident,
+leave it undone. If it was deliberate, redo the change as described
+below.
+
+@findex undo-only
+ Any command other than an undo command breaks the sequence of undo
+commands. Starting from that moment, the previous undo commands
+become ordinary changes that you can undo. Thus, to redo changes you
+have undone, type @kbd{C-f} or any other command that will harmlessly
+break the sequence of undoing, then type undo commands again. On the
+other hand, if you want to resume undoing, without redoing previous
+undo commands, use @kbd{M-x undo-only}. This is like @code{undo}, but
+will not redo changes you have just undone.
+
+@cindex selective undo
+@kindex C-u C-x u
+ Ordinary undo applies to all changes made in the current buffer. You
+can also perform @dfn{selective undo}, limited to the region.
+
+ To do this, specify the region you want, then run the @code{undo}
+command with a prefix argument (the value does not matter): @kbd{C-u
+C-x u} or @kbd{C-u C-_}. This undoes the most recent change in the
+region. To undo further changes in the same region, repeat the
+@code{undo} command (no prefix argument is needed). In Transient Mark
+mode (@pxref{Transient Mark}), any use of @code{undo} when there is an
+active region performs selective undo; you do not need a prefix
+argument.
+
+ Some specialized buffers do not make undo records. Buffers
+whose names start with spaces never do; these buffers are used
+internally by Emacs and its extensions to hold text that users don't
+normally look at or edit.
+
+@vindex undo-limit
+@vindex undo-strong-limit
+@vindex undo-outer-limit
+@cindex undo limit
+ When the undo records for a buffer becomes too large, Emacs
+discards the oldest undo records from time to time (during garbage
+collection). You can specify how much undo records to keep by
+setting three variables: @code{undo-limit}, @code{undo-strong-limit},
+and @code{undo-outer-limit}. Their values are expressed in units of
+bytes of space.
+
+ The variable @code{undo-limit} sets a soft limit: Emacs keeps undo
+data for enough commands to reach this size, and perhaps exceed it,
+but does not keep data for any earlier commands beyond that. Its
+default value is 20000. The variable @code{undo-strong-limit} sets a
+stricter limit: a previous command (not the most recent one) which
+pushes the size past this amount is itself forgotten. The default
+value of @code{undo-strong-limit} is 30000.
+
+ Regardless of the values of those variables, the most recent change
+is never discarded unless it gets bigger than @code{undo-outer-limit}
+(normally 3,000,000). At that point, Emacs discards the undo data and
+warns you about it. This is the only situation in which you cannot
+undo the last command. If this happens, you can increase the value of
+@code{undo-outer-limit} to make it even less likely to happen in the
+future. But if you didn't expect the command to create such large
+undo data, then it is probably a bug and you should report it.
+@xref{Bugs,, Reporting Bugs}.
+
+ The reason the @code{undo} command has three key bindings, @kbd{C-x
+u}, @kbd{C-_} and @kbd{C-/}, is that it is worthy of a
+single-character key, but @kbd{C-x u} is more straightforward for
+beginners to remember and type. Meanwhile, @kbd{C--} on a text-only
+terminal is really @kbd{C-_}, which makes it a natural and easily
+typed binding for undoing.
+