A letter’s creases can tell you how often it was taken out again.

Paper keeps a record of handling. A letter folded once and filed looks different from one that has been opened and refolded until the creases have gone soft and slightly torn, and that wear is the only evidence of how much it was needed. Nobody writes down how many times they reread something. The paper does it for them.

If you find an old letter in that state, consider who wore it out and what they were going back for. And if something someone wrote to you still helps, tell them. People rarely know which of their sentences got kept.