The Jargon File (version 4.4.7, 29 Dec 2003):

space-cadet keyboard
 n.

    A now-legendary device used on MIT LISP machines, which inspired several
    still-current jargon terms and influenced the design of EMACS. It was
    equipped with no fewer than seven shift keys: four keys for bucky bits
    (?control?, ?meta?, ?hyper?, and ?super?) and three regular 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 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            |lowercase l                    |
    |-------------+-------------------------------|
    |shift-L      |uppercase L                    |
    |-------------+-------------------------------|
    |front-L      |?                              |
    |-------------+-------------------------------|
    |front-shift-L|?                              |
    |-------------+-------------------------------|
    |top-L        |? (front and shift are ignored)|
    +---------------------------------------------+

    And of course each of these might also be typed with any combination of the
    control, meta, hyper, and super keys. On this keyboard, you could type over
    8000 different characters! This allowed the user to type very complicated
    mathematical text, and also to have thousands of single-character commands
    at his disposal. The keyboard of the Symbolics Lisp machine was a
    simplified version, lacking Top and Front keys, that could only send about
    2000 characters.

    Many hackers were actually willing to memorize the command meanings of that
    many characters if it reduced typing time (this attitude obviously shaped
    the interface of EMACS). Other hackers, however, thought having that many
    bucky bits was overkill, and objected that such a keyboard can require
    three or four hands to operate. See bucky bits, cokebottle, double
    bucky, meta bit, quadruple bucky.

    [symbolics-]

    Simplified Symbolics version of the space-cadet keyboard

    (Some relatively bad photographs of the earlier, more elaborate version are
    available on the Web.).

    Note: early versions of this entry incorrectly identified the space-cadet
    keyboard with the Knight keyboard. Though both were designed by Tom Knight,
    the latter term was properly applied only to a keyboard used for ITS on the
    PDP-10 and modeled on the Stanford keyboard (as described under bucky bits
    ). The true space-cadet keyboard evolved from the first Knight keyboard.

    [73-05-19]

    An early space-cadet keyboard


The Free On-line Dictionary of Computing (30 December 2018):

space-cadet keyboard

   A now-legendary device used on MIT Lisp machines, which
   inspired several still-current jargon terms and influenced the
   design of Emacs.  It was equipped with no fewer than *seven*
   shift keys: four keys for bucky bits ("control", "meta",
   "hyper", and "super") and three like regular 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 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		lowercase l

    shift-L	uppercase L

    front-L	lowercase lambda

    front-shift-L	uppercase lambda

    top-L		two-way arrow

   (front and shift are ignored) And of course each of these
   might also be typed with any combination of the control, meta,
   hyper, and super keys.  On this keyboard, you could type over
   8000 different characters!  This allowed the user to type very
   complicated mathematical text, and also to have thousands of
   single-character commands at his disposal.  Many hackers were
   actually willing to memorise the command meanings of that many
   characters if it reduced typing time (this attitude obviously
   shaped the interface of Emacs).  Other hackers, however,
   thought that many bucky bits was overkill, and objected that
   such a keyboard can require three or four hands to operate.

   See cokebottle, double bucky, meta bit, quadruple
   bucky.

   Note: early versions of this entry incorrectly identified the
   space-cadet keyboard with the "Knight keyboard".  Though both
   were designed by Tom Knight, the latter term was properly
   applied only to a keyboard used for ITS on the PDP-10 and
   modelled on the Stanford keyboard (as described under bucky
   bits).  The true space-cadet keyboard evolved from the Knight
   keyboard.

   [Jargon File]

   (1994-12-05)