Rule of Premises. You may write down a premise at any point in a proof.
The second rule of inference is one that you'll use in most logic proofs. It is sometimes called modus ponendo ponens, but I'll use a shorter name.
Modus Ponens. If you know P and $P ifthen Q$ , you may write down Q.
In the rules of inference, it's understood that symbols like "P" and "Q" may be replaced by any statements, including compound statements.