Blue Book = B = Blue Glue

blue box

n. 1. obs. Once upon a time, before all-digital switches made it possible for the phone companies to move them out of band, one could actually hear the switching tones used to route long-distance calls. Early phreakers built devices called `blue boxes' that could reproduce these tones, which could be used to commandeer portions of the phone network. (This was not as hard as it may sound; one early phreak acquired the sobriquet `Captain Crunch' after he proved that he could generate switching tones with a plastic whistle pulled out of a box of Captain Crunch cereal!) There were other colors of box with more specialized phreaking uses; red boxes, black boxes, silver boxes, etc. There were boxes of other colors as well, but the blue box was the original and archetype. 2. n. An IBM machine, especially a large (non-PC) one.

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.