The space-cadet keyboard is a keyboard used on MIT Lisp machines and designed by Tom Knight, which inspired several still-current jargon terms in the field of computer science and influenced the design of Emacs. It was inspired by the Knight keyboard (also developed by Tom Knight), which was developed for the Knight TV system, used with MIT's Incompatible Timesharing System.
The Symbolics-labeled version shown here was only used with the LM-2, which was Symbolics’s repackaged version of the MIT CADR. Later Symbolics systems used a greatly simplified keyboard that retained only the basic layout and the more commonly used function and control keys from the space-cadet keyboard.
The space-cadet keyboard was equipped with eight modifier keys: four keys for bucky bits ("control", "meta", "hyper", and "super"; the latter two of which were introduced by this keyboard) and three shift keys, called "shift", "top", and "front". Many keys had three symbols on them: a letter and a symbol on the top, and a Greek letter on the front. For example, the "L" key had an "L" and a two-way arrow on the top, and the Greek letter lambda ("Λ") on the front. By pressing this key with the right hand while playing an appropriate "chord" with the left hand on the shift keys, you could get the following results:
|L||l (lowercase L)|
|⇧ Shift+L||L (uppercase L)|
|Front+L||λ (lowercase lambda)|
|Front+⇧ Shift+L||Λ (uppercase lambda)|
|Top+L||⇔ (two-way arrow)|
Each of these might, in addition, be typed with any combination of the "control", "meta", "hyper", and "super" keys. On this keyboard, it is possible to type over 8,000 different characters. This allowed the user to type very complicated mathematical text, and also to have thousands of single-character commands at their disposal. Many users were willing to memorise the command meanings of that many characters if it reduced typing time (this attitude shaped the interface of Emacs). Other users, however, thought that so many bucky bits was overkill, and objected to this design on the grounds that such a keyboard can require three or four hands to operate.
This keyboard included a Macro Key which had limited application support. It also included four Roman Numeral keys (I, II, III, and IV) which allowed for easy interaction with lists of four or fewer choices.
See also 
This article is based in part on the Jargon File, which is in the public domain.