# Lemma/Proposition/Theorem, which one should we pick?

This is something that confuses me.
I have read a few mathematical texts and they often seem to use Lemma/Proposition/Theorem if they have a particular statement.

Now, which one to use? A lemma can be something you need to prove a more important theorem, but then what about Fatou’s Lemma?

When to pick Proposition or Theorem?