The Top Key

The Top key first appeared on SAIL Keyboard around 1970.

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  SAIL Keyboard

It is like the Shift key, used to input Math Symbols ∞ .

Then it appeared on Lisp keyboards

• 
  Knight Keyboard

• 
  Space Cadet

Seems, after the Space Cadet, the Top key disappeared. But the concept is picked up, as Apple's ⌥ option key [see Mac Keyboard Viewer] and AltGraph key. [see Alt Graph Key, Compose Key, Dead Key]