The Probert Encyclopaedia of General Information

ENVELOPE

Envelopes are the paper covers that inclose letters or notes. They became common shortly after the introduction of the penny postage system in 1840 and were at first made chiefly by hand, but since the end of the 19th century were not only shaped, but folded, gummed, etc, by machinery. Envelopes for letters were first mentioned by Jonathan Swift in 1726.
Research Envelope