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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 the left-hand part of the screen, red and blue scroll up and down respectively, and yellow opens a page;
  • in the document, red selects a single character, yellow selects a word, and blue extends the selection (using characters or words depending on what’s already been selected) — this is somewhat similar to Emacs’ mark and point;
  • in the “line bar” (between the scroll bar and the text area), yellow selects a paragraph, and blue extends the selection;
  • with the “Window” command, blue creates a new window (with a new document), yellow splits the current window (giving two views of the current document), red moves the window;
  • with the “Kill” command, red kills a window and gives its space to the window above, blue to the window below, and yellow clears the window but doesn’t kill it.

In Laurel, the messaging program:

  • when invoking commands, red invokes a command potentially with confirmation, blue invokes a command with implicit confirmation, and yellow cancels some commands;
  • in the scroll bar, red and blue work as in Bravo, and yellow moves around within sections (the scroll bar is somewhat more complex than their modern incarnations);
  • in the message list, red and blue select messages;
  • etc.

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