The UNDO function allows for the last user action(s) to be "undone". The presence of an UNDO functions encourages users to explore the functionality of a system.
 

Guidelines:

• Even when a general UNDO function is available, special UNDO functions should nevertheless be preserved
• E.g., delete left, delete right, second click on selected menu items to undo the selection.
• They can be activated more quickly and already became a convention.

• If an UNDO function is available, everything should be undoable, if possible
  • It is difficult for users to understand that some actions in a program that have local effects only (like saving a document to the harddisk, or overwriting a document) cannot be undone.
  • [In contrast, users understand easily that actions that have external effects (like sending an email message) cannot be undone.]

• Multiple UNDOs should be possible

• UNDO of UNDO:
• Toggle UNDO: easy for beginners
• Linear sequence of UNDOs and REDOs
  Any other keyboard input expunges the REDO memory

• If confirmation for UNDO or Cancel is requested, it must be carefully formulated (avoid double negation, e.g. double "cancel")