Envelope is a piece of folded, sealed paper. It serves as a wrapper to hold letters and other papers to be mailed, or acts as protection against loss or damage. Envelopes are made of a single sheet of paper folded so that one flap remains open. This flap is gummed so the envelope may be sealed after the letter or papers have been inserted. Self-sealing envelopes make use of a gum that need not be moistened to stick.
Envelopes were first manufactured in 1839 by a New York City man named Pierson. Before that time, all letters were folded so that a blank portion of the paper could be used for the address. The edges of the paper were sealed with sealing wax. For about 10 years after envelopes were invented, they were cut, folded, and pasted by hand. A worker could make about 3,000 envelopes a day. The first successful machine for making envelopes was patented in 1849 by J. K. Park and C. S. Watsen of New York City. A greatly improved envelope machine, patented in 1898 by John Ames Sherman, lowered the cost of making gummed envelopes.
Envelopes are numbered from 5 to 14 according to size. The No. 10 envelope is the most popular size for business letters. It measures 91/2 inches (24.1 centimeters) long and 41/8 inches (10.5 centimeters) wide.
Some standardization of envelope sizes is necessary for the efficient operation of mechanical mail-sorting devices. The U.S. Postal Service requires that envelopes sent through the mail be rectangular and no smaller than 5 inches (12.7 centimeters) long and 31/2 inches (8.9 centimeters) wide. The Universal Postal Union, a United Nations agency, has a metric size range to make the handling of international mail more efficient. Envelopes should be at least 14 centimeters (51/2 inches) long and 9 centimeters (31/2 inches) wide, but no more than 23.5 centimeters (91/4 inches) long and 12 centimeters (42/3 inches) wide.
