19-inch rack standard.

[Jim Haynes]

I've wondered about that too.  Only thing I know is that it originated
with Western Electric, and that you can get ten relays on a panel that
wide.  Maybe they just picked an arbitrary size that happened to fit
ten relays and then used it for everything.  The main manual switchboards
used a pair of relays for every line.  Then later on they introduced a
23 inch rack.  Might be easier to find out where that number comes from,
since it's more recent.  Automatic Electric (a big supplier to the non-Bell
telephone companies) and Western Union had standards of their own which
are not 19 inches nor 23 inches, tho they probably used some 19-inch racks
as well since it was sort of an industry standard.

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