A letter of recommendation asks one person’s credibility to travel with another person’s application.

A recommendation sends one person’s credibility along with another person’s application. The reader cannot assess the candidate, so they assess the recommender and transfer the judgement, which means the letter is a loan of standing rather than a description of a person. That is why who writes it matters as much as what it says.

If you are asked for one, take it seriously, since you are lending something real. Write specifically or decline honestly, as a vague letter does more harm than none. And when you need one, ask someone who can describe you concretely rather than someone impressive who barely knows you.