The original GUI invented at Xerox Parc, used a three-button mouse.
What did each of the three buttons do?
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On the Alto, the buttons’ functions vary with the program currently being used and the location of the cursor. The Users’ Handbook describes the functions in the default applications, using colours for the buttons: red for the left button (or top button), yellow for the middle button, blue for the right button (or bottom button).
In Bravo, the text document editor, the buttons are used as follows:
In Laurel, the messaging program: