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 es that could reproduce these tones, which could be
used to commandeer portions of the phone network. 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  was the
original and archetype. 2. n. An IBM machine, especially a large (non-PC)
one.
