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Envelopes have a long history... They were invented by the Chinese around 3500BC as a way of keeping royal communications secret. These envelopes were not made out of paper — they were hollow clay spheres. The secret message would be dropped into the sphere and sealed with clay, to be broken upon delivery. This would both keep the letter from prying eyes and also make any breach or tampering immediately obvious. In these ancient times, the average citizen couldn't read or write, clay envelopes were not often used by common people.
By the 2nd century BC, paper envelopes were developed, and used in China to store gifts of money. To this day in Japan, special envelopes are sold to put gifts for bereaved relatives in.
It wasn't until 1845 that the first automatic envelope making machine was invented, and nearly 50 years after that before machines could "pre-gum" the envelopes for convenience, giving rise to the famous lick gesture. Prior to the development of the pregummed envelope, glue or wax was applied manually to seal the envelope.
Today, envelopes are available in dozens of standard sizes, and custom sizes can be easily ordered. Inkjet or laser printers as used in the average home or office (or home office) can be used to print legible, deliverable envelopes. With inkjet printers, care must be taken to use waterproof ink so that wet weather doesn't smear the address, potentially delaying or even preventing delivery. In 1902, Chicago inventor Americus F. Callahan was granted a patent for the first window envelope, with a clear inlay on the envelope's front to allow an address printed on the enclosure itself to be seen. Callahan recommended making window envelopes out of manila or black paper to for increased privacy. Window envelopes are very commonly used in commercial mailings, but seldom by individuals.
Source: Wikipedia