Undo

Undo is a command in many computer programs. It erases the last change done to the document reverting it to an older state. In some more advanced programs such as graphic processing, undo will negate the last command done to the file being edited.

The opposite of undo is redo. The redo command reverses the undo or advances the buffer to a more current state.

In most Windows applications, the Undo command is activated by pressing the Ctrl+Z or Alt+Backspace keybindings. In all Macintosh applications, the Undo command is activated by pressing Command-Z. The common command for Redo on Microsoft Windows systems is Ctrl+Y or Ctrl+Shift+Z. The common command for Redo on Apple Macintosh systems is Command-Shift-Z.

The ability to undo an operation on a computer was independently invented multiple times, as a response to how people used computers.

The File Retrieval and Editing System, developed starting in 1968 at Brown University, is reported to be the first computer-based system to have had an "undo" feature.

Warren Teitelman developed a Programmer's Assistant as part of BBN-LISP with an Undo function, by 1971.

The Xerox PARC Bravo text editor had an Undo command in 1974.Behavioral Issues in the Use of Interactive Systems, a 1976 research report by Lance A. Miller and John C. Thomas of IBM, noted that "It would be quite useful to permit users to 'take back' at least the immediately preceding command (by issuing some special 'undo' command)." The programmers at the Xerox PARC research center assigned the keyboard shortcut Ctrl-Z to the undo command, which became a crucial feature of text editors and word processors in the personal computer era.

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