Folded Envelopes

If you are not into envelope and letter folding you may wonder why one should go to the trouble of folding up an envelope when you could just buy one from a stationery store. Well, first off, it is the very nature of hobbies that they tend not to be entirely practical. In spite of that, hand folding letters and envelopes is one of those rare intersections of decoration and practicality, where paper folding produces the satisfaction of making something useful and novel. The folds can be extremely clever in their attempts to reach a number of basic goals: to make the largest possible envelope from a given piece of paper, to latch or remain sealed with out need of adhesive, to meet postal requirements and to be attractive. One of the "Holy Grails" of envelope and letter folding is to create a fold which not only latches, but can't be unfolded--sealing it shut like a glued envelope. No one seems to have managed this feat, so far.

[ Has the internet figured it out yet SPOILDER: probably... ]