All lab GUIs starting from Engelbart’s AUGMENT had 3-button mouses, including the Xerox Alto. The researchers and engineers using the Alto found that the mouse

All lab GUIs starting from Engelbart’s AUGMENT had 3-button mouses, including the Xerox Alto. The researchers and engineers using the Alto found that the mouse buttons were confusing, even this small group couldn’t remember the combinations in each application, and it lead to many inconsistencies between apps. Each button had a very precise function, and they were as much modal shortcuts than direct manipulation buttons. Even Xerox tried to reduce it to one button for the Star (their first commercial GUI computer), but from their own published account they couldn’t find a way and the Star shipped with a 2-button mouse. Remember, at the time most people targeted by Apple (or other computer manufacturers) had never used a computer, and the people who did use a computer never used a mouse! (Except for the PARC researchers and some other researchers, so maybe 2000 people worldwide) It’s people coming from Xerox to Apple who were the most interested in having a one button mouse! They knew the 3-button mouse would confuse users and was entirely unnecessary for most users (as it is today, the secondary click being reduced to just a convenient shortcut, nothing more) A previous comment about the 2-button Star and how these buttons were actually used: https://news.ycombinator.com/item?id=31750283 https://news.ycombinator.com/item?id=31750283