Chapter 12: Understanding Undo

Following the user's mental model means absolving the user of blame.

Everything a user does is something he or she considers to be valid and reasonable.

Undo enables exploration. You would be more likely to explore a dark cave if you knew you could just press a button and be returned to safety.

Types and Variants of Undo:
Actions that include a data component are called incremental actions.
Actions that act on data, but neither add or delete data, are procedural actions (e. g. paragraph formatting)

Blind undo - not readily apparent what the undo did.
Explanatory is better, telling user what will take place if choosing undo.