For centuries after paper became common letter writing material in the mid 14th century, documents were usually sealed (a.k.a. locked) for transport and storage. Sealing consisted of various methods of folding the letter, to protect its interior from dirt and stain, and binding with wax, ribbon, string, ..., to render tamper-evident.
Before the first public postal service (1775.07.26 2nd Continental Congress), all written messages were delivered by courier. Nobility, militaries, and large businesses, assigned vassels, soldiers, and employees, especially elderly or infirm, to courier duty. It became custom for lesser individuals and businesses to place (fr. poste) their sealed letters with address in a box on a pole (ang. post) by the road leading out of town. Interested travellers would examine these boxes and take with them letters going to places on their route. Delivering a letter was customarily rewarded by the recipient with lodging and food for the night, and/or money.
Letterlocking, sometimes synonymous with letter sealing, usually referred to more elaborate means of making letters tamper-evident, especially if the evidence of tampering was only apparent to the sender and/or receiver, and not apparent to the tamperer.
Today, the USPS identifies such sealed letters as folded self-mailers.
Method of folding is generally chosen by Aspect Ratio (i.e. height/width).
A bone scorer is a tool for making sharp and accurate folds. A bone scorer got its name from being typically manufactured from a flat bone. In the 19th and 20tch century, bone scorers were more often made of wood sanded smooth, or later plastic. Ideally, a bone scorer is about 6in long, about 1/4in to 1in wide, and about 1/8in thick tapering to about 1/32in over the last 1in at the ends. Ideally, a bone scorer is sanded smooth, so that it can be rubbed under pressure against paper without tearing or cutting.
If you don't have a bone scorer, you can use
This was the most common method of folding letters in the United States and United Kingdom for at least two centuries before Wold War II. The resulting 3x5in size, set the standard for the first index cards in 1760, by Carl Linnaeus, "the father of modern taxonomy", and "postal size" library catalog cards published by the American Library Association from 1877.
[instructions]This fold doesn't protect its contents as well as most other sealed letters (e.g. usletter to postal), but it requires no precision, and applies to any aspect ratio.
In 19th century America, personal documents, folded in this fashion would be carried in a billfold, along with banknotes/cash (then 3x7in). When the 1890 census needed a format for tabulating, Hollerith chose the 3.125x7.125in size to fit cash drawers then in use. For the next century, even after federal reserve notes shrunk in the 1920s, the IBM computer card remained 3x7in.
[instructions]This fold sacrifices the left side of its contents to potential damage by the seal. Consequently, it lends itself to letterlocking techniques that are more tamper evident or even tamper resistant. The seal can simply be cut off, a precursor to modern perforated self-mailers.
After removing the seal, this fold can also be unfolded (and, with practice, refolded) one-handed.