Push the envelope

[Paul McFedries]

After some canny hints from Barry Popik and Leslie Savan, I think I've
tracked down the origin of "pushing (the edge of) the envelope." It seems
that the envelope metaphor is derived from mathemetics, where an "envelope"
is the outer boundary of a set of curves. In aeronautical engineering,
flight characteristics such as air speed, rate of climb and descent, and
rate of directional change can all be represented graphically as curves. So
the outer boundary of those curves -- the envelope -- represents the
theoretical limit of the aircraft's performance.

Paul
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