Why is going beyond the known limits called “pushing the envelope”?

“Pushing the envelope” is an aviation expression that refers to how test pilots received instructions to challenge the known limits of flight. These instructions, if not a death sentence, were very often a flirtation with disaster. The gravity of issuing such an order was understood but not spoken. Instead, the impersonal assignment came within an envelope, silently slid or pushed across a desk from one man to another.

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