Everyone knows Ctrl+Z steps back. But typing UNDO brings a long prompt with six options, typing U just does it without asking, and typing REDO twice does nothing the second time. Three things that look like one turn out to be three.
This article opens the AutoCAD 2027 interface file to find what those two shortcuts actually run, then measures the result on the machine. One finding took two rounds of measuring before we believed it. Undo in AutoCAD gets used hundreds of times a day, so understanding it properly saves real minutes. The Institute runs an AutoCAD course for construction covering the groundwork.
Undo in AutoCAD: how U and UNDO differ
These are two separate commands, not an abbreviation of one another, and that is the first misunderstanding to clear.
U steps back exactly one operation and ends, asking nothing. Drawing six circles and called U three times: the object count went 6 → 5 → 4 → 3, one step each time.
UNDO opens a whole set of options. It prints a status line before asking, copied verbatim from the 2027 release:
Current settings: Auto = On, Control = All, Combine = Yes, Layer = Yes
Enter the number of operations to undo or [Auto/Control/BEgin/End/Mark/Back] <1>:
The default in the angle brackets is 1, so pressing Enter behaves exactly like U. Type a number instead and it steps back that many operations at once. With 8 objects on screen we typed UNDO then 3, and exactly 5 remained.
Autodesk describes the command in one line on the UNDO page for the 2027 release: "Reverses the effect of commands".
UNDO, REDO and OOPS all have no alias and no button on the default ribbon. Only U appears in the Edit menu, and MREDO sits at Edit → Redo. Setting up aliases is covered in our article on command aliases in AutoCAD.
The OOPS command is a relative few people know. It restores the objects just erased without reversing anything done since, which is quite unlike U. In exactly that case: draw three circles, erase one, draw two more, then type OOPS. The count went from 4 to 5 — the erased circle returned while both new circles stayed. Press U instead and it is the new circles that disappear. Editing in bulk without resorting to undo is covered in our article on editing text in bulk.
What the undo and redo shortcuts really call
This is the part we have not seen written anywhere, and it explains why REDO does nothing the second time.
The acad.cuix file of the 2027 release defines 43 keyboard accelerators. Unpack it and read, and two lines stand out:
| Key | What it actually runs |
|---|---|
Ctrl+Z | _u |
Ctrl+Y | ^C^C_mredo 1 |
So the two shortcuts are not symmetrical. Ctrl+Z calls U, while Ctrl+Y does not call REDO at all — it calls MREDO with an argument of 1.
Three presses of Ctrl+Z: 5 → 4 → 3 → 2 circles. Three presses of Ctrl+Y straight afterwards: 2 → 3 → 4 → 5. Both directions ran all three times.
The opening figure shows the two ends of that test: five circles, and exactly two after three presses of Ctrl+Z.
The REDO command only reverses one step
Here is the odd part. Type REDO at the command line and it restores one step: 3 → 4 objects. Type REDO a second time and nothing happens. We tried five times in a row and the count did not move.
At first we assumed the command was being swallowed. But Autodesk's documentation says exactly that: REDO "reverses the effects of a single UNDO or U command" — one reversal only, and it does not repeat.
For more than one there is MREDO, which Autodesk describes as reversing "the effects of several previous UNDO or U commands". We typed MREDO then 2, and the count went from 4 to 6, redoing two steps at once.
Put the two together and everything fits: typing redo works once, while Ctrl+Y works as many times as you like, because each press is a fresh MREDO 1. Anyone who has typed redo and concluded it was broken now has the reason.
The Back option, and why it deserves caution
Of the six options on UNDO, the two most dangerous sit side by side.
Autodesk describes the pair in one sentence: "Mark places a mark in the undo information. Back undoes all the work done back to this mark". You drop a marker, carry on working, then one Back returns everything to that marker.
The trouble is using Back when no marker has been placed. We tried it on a blank drawing and the software asked exactly one question:
This will undo everything. OK? <Y>
The <Y> at the end is the default. Anyone in the habit of pressing Enter to clear prompts has just agreed to lose the entire session. This is one of the few places in AutoCAD where a reflex Enter does real damage.
The BEgin and End options are gentler. Autodesk notes they group "a sequence of actions into a set". Open a group, do a run of work, close the group, and the whole run steps back with a single U. Useful when running a sequence you may want to reverse as one piece.
What the UNDOCTL variable tells you
To see the state of the undo system without running a command, read the UNDOCTL variable. Autodesk defines it briefly: "Displays the options used in the UNDO command".
it read 53. It is a bitcode, so it has to be split: 53 = 1 + 4 + 16 + 32.
| Bit | What Autodesk says |
|---|---|
| 1 | "UNDO is turned on" |
| 4 | "Auto is turned on" |
| 16 | "Zoom and pan operations are grouped as a single action" |
| 32 | "Layer property operations are grouped as a single action" |
Those four lines decode to exactly the status line the UNDO command printed: Auto = On, Control = All, Combine = Yes, Layer = Yes. Number and words agree, so reading the variable is enough.
Bit 16 explains something that often irritates people: zoom in and out repeatedly, press Ctrl+Z, and the whole run of zooms steps back as one rather than individually. That is deliberate, not a fault.
Bit 2 means only a single command can be undone, which happens when someone has set UNDO to its One mode. On a drawing where Ctrl+Z only ever steps back once, check UNDOCTL first.
One consequence is worth knowing for office templates. Because the undo settings travel with whoever set them, a drawing arriving from a consultant can behave differently from your own files without anything looking wrong on screen. Reading the variable takes a second and removes the guesswork, which beats assuming the software has developed a fault.
Three things tutorials leave out
| What people assume | What the 2027 release actually does |
|---|---|
Ctrl+Y is the REDO command | No. The interface file shows it runs ^C^C_mredo 1 |
Typing REDO repeatedly redoes several steps | Only one; for more, use MREDO |
U and UNDO are the same thing | U steps back once; UNDO has six options, one of which wipes the session |
Three things cover nearly all of undo in AutoCAD: U for ordinary work, UNDO with a number when a run has to go, and Back avoided unless a marker was set with Mark.
On a large drawing one habit is worth forming. Before anything risky — a bulk text edit, an unfamiliar lisp, importing an outside drawing — type UNDO then M to drop a marker. If the result is wrong, UNDO then B returns everything to that point, which is far tidier than pressing Ctrl+Z a few dozen times while counting in your head.
Frequently asked questions
What is the undo shortcut in AutoCAD?
Ctrl+Z. Reading the interface file, it runs the U command, stepping back exactly one operation per press.
Why does typing REDO a second time do nothing?
Because REDO reverses only one undo. Use MREDO with a number, or simply press Ctrl+Y repeatedly.
How do I step back ten operations at once?
Type UNDO, then 10, then Enter.
Ctrl+Z only steps back once on my drawing. Why?
Check the UNDOCTL variable. If it carries bit 2, the mode has been set to One; type UNDO and use the Control option to change it.
Is the Back option dangerous?
Yes. With no marker set, it asks This will undo everything. OK? <Y> and the default answer is yes.
Why does undo step back a whole run of zooms?
Bit 16 of UNDOCTL: zoom and pan operations are grouped into a single action.