Push the envelope

[Paul McFedries]

I'm trying to find some background on the idiom "push the envelope" (to
increase the operating capabilities of a system). I have some vague
recollection that it originated with aircraft and had something to do with
the speed of sound and that Tom Wolfe talked about it in The Right Stuff.
However, my wasted youth prevents a complete recall. In particular, I'm
interested in why the word "envelope" is used.

Thanks a bunch.

Paul
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