01 / THE IDEA
What does it mean?
Give people a clear way to leave an action they started by mistake or reverse one they have already taken. Use Cancel before a change takes effect and Undo after a reversible change.
02 / IN PRACTICE
Make it concrete.
Illustrative example: After archiving a conversation, a toast offers Undo for a short period.
03 / WHAT TO WATCH FOR
A common mistake.
Asking for confirmation on every harmless action instead of allowing undo.
04 / WHEN IT HELPS
When should you use it?
When changes are frequent, easy to make accidentally, or reversible.
05 / FURTHER READING
Explore the source.
Nielsen Norman Group — Usability Heuristics ↗A reference for further learning, not an endorsement or verbatim quotation.