One of jed's nicest features is the ability to undo nearly any change that occurs within a buffer at the touch of a key. If you delete a word, you can undo it. If you delete 10 words in the middle of the buffer, move to the top of the buffer and randomly make changes, you can undo all of that too.
By default, the undo function is bound to the key CTRL-/SMALL>_
(Ascii 31). Since some terminals are not capable of generating this
character, it is also bound to the key sequence CTRL-X u.
Due to the lack of virtual memory support on IBMPC systems, the
undo function is not enabled on every buffer. In particular, it
is not enabled for the *scratch* buffer. However, it is enabled
for any buffer which is associated with a file. A ``plus'' character on
the left hand side of the status line indicates that undo is enabled for
the buffer. It is possible to enable undo for any buffer by using the
toggle_undo function.