The undo command reverses 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 back as the buffer's records go. Usually, each editing
command makes a separate entry in the undo records, but some commands
query-replace divide their changes into multiple
entries for flexibility in undoing. Consecutive character insertion
commands are usually grouped together into a single undo record, to
make undoing less tedious.
To begin to undo, type C-/ (or its aliases, C-_ or C-x u)1. This undoes the most recent change in the buffer, and moves point back to where it was before that change. Consecutive repetitions of C-/ (or its aliases) undo earlier and earlier changes in the current buffer. If all the recorded changes have already been undone, the undo command signals an error.
Any command other than an undo command breaks the sequence of undo commands. Starting from that moment, the entire sequence of undo commands that you have just performed are themselves placed into the undo record. Therefore, to re-apply changes you have undone, type C-f or any other command that harmlessly breaks the sequence of undoing; then type C-/ one or more times to undo some of the undo commands.
Alternatively, if you want to resume undoing, without redoing
previous undo commands, use M-x undo-only. This is like
undo, but will not redo changes you have just undone.
If you notice that a buffer has been modified accidentally, the easiest way to recover is to type C-/ repeatedly until the stars disappear from the front of the mode line (see Mode Line). 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 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 above.
Alternatively, you can discard all the changes since the buffer was last visited or saved with M-x revert-buffer (see Reverting).
When there is an active region, any use of
selective undo: it undoes the most recent change within the
region, instead of the entire buffer. However, when Transient Mark
mode is off (see Disabled Transient Mark), C-/ always
operates on the entire buffer, ignoring the region. In this case, you
can perform selective undo by supplying a prefix argument to the
undo command: C-u C-/. To undo further changes in the
same region, repeat the
undo command (no prefix argument is
Some specialized buffers do not make undo records. Buffers whose names start with spaces never do; these buffers are used internally by Emacs to hold text that users don't normally look at or edit.
When the undo information for a buffer becomes too large, Emacs discards
the oldest records from time to time (during garbage
collection). You can specify how much undo information to keep by
setting the variables
undo-outer-limit. Their values are expressed in bytes.
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 80000. The variable
undo-strong-limit sets a
stricter limit: any previous command (though not the most recent one)
that pushes the size past this amount is forgotten. The default value
undo-strong-limit is 120000.
Regardless of the values of those variables, the most recent change
is never discarded unless it gets bigger than
(normally 12,000,000). 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
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.
See Reporting Bugs.
 Aside from C-/, the
undo command is
also bound to C-x u because that is more straightforward for
beginners to remember: ‘u’ stands for “undo”. It is also bound
to C-_ because typing C-/ on some text terminals actually