I am very grateful to the person who created the question, which reminded me of the scene my father taught me when I was a child.

Letter to elders: When the letter paper is folded at the end, the beginning of the letter paper (the part with the honorific address to the elders) must be longer than the part of the letter paper with the letter writer's signature. Meaning: The elder is on top. Show respect for elders.

Letter to peers: When the letter paper is folded at the end, the letter paper can be folded in half from 1/2, and the height of the letter paper at the beginning and end should be equal. Meaning: equality among peers.

Letter to juniors: When the letter paper is folded at the end, the part of the letter paper with the writer's signature is longer than the folded part at the beginning. Meaning: The person signing the name is an elder and should be on top. Shows dignity in front of juniors.