National Council of Teachers of Mathematics
The Math Forum: Ask Dr. Math: Building Two Column Proofs
Use this site to find details of how to construct a two column proof. Dr. Math responds to a question posed by a student and details the reasons for the order of steps and shows two different ways to do the same proof.
National Council of Teachers of Mathematics
The Math Forum: Ask Dr. Math: Order of a Proof
The site gives a diagram of steps to a proof. Dr. Math explains why the step order of a proof is important.