Why were Soviet soldiers' letters from the front lines triangular?

Soldiers folded them so that the text was on the inside and wrote the sender's name and recipient's address on the outside. A postage stamp was not required.

Glue was also not used for these so-called "soldiers’ triangles". After folding the opposite sides of the sheet, a strip of paper remained that served as a kind of flap — it was tucked inside the triangle.

A triangular letter didn’t get as crumpled in the mail carrier's bag; it could be easily unfolded and then folded back up again. This was important, because all correspondence was screened by military censors. They would black out unit numbers, names of commanders or descriptions of the terrain where the soldier fought and then stamp the letters with their seal.

There is a monument to the "soldiers’ triangle" in the Volgograd Region. Engraved on a triangular marble slab is the original text of a letter written by Dmitry Petrakov, a participant in the Defense of Stalingrad, to his daughter Lyudmila.