Setting a bucky bit changes the output character. A bucky bit allows the user to type a wider variety of characters and commands while maintaining a reasonable number of keys on a keyboard.
In ASCII, the bucky bit is usually the 8th bit (also known as meta bit). However, in older character representations wider than 8 bits, more high bits could be used as bucky bits. In the modern X Window System, bucky bits are bits 18-23 of an event code.
The term was invented at Stanford and is based on Niklaus Wirth's nickname "Bucky". Niklaus Wirth was first to suggest an EDIT key to set the eighth bit of a 7-bit ASCII character sometime in 1964 or 1965. 
Bucky bits were used heavily on keyboards designed by Tom Knight in MIT, including space-cadet keyboards used on LISP machines. These could contain as many modifier keys as: SHIFT, CTRL, META, HYPER, SUPER, TOP, FRONT, and GREEK. 
See also 
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