This TrueFit poster from Charlotte Pipe explains how pipe and fittings work together.

You've seen it before. A joint in the plumbing system leaks, causing costly damage and construction delays. The pipe manufacturer blames it on the fitting maker, and the fitting maker blames it on the pipe manufacturer. If this hasn't happened to you, it could if you use pipe from one company and fittings from another. Too many plumbing contractors think all pipe and fittings are the same. But unfortunately, it is possible for a mechanic to receive pipe and fittings that do not fit together properly, even though they meet all of the applicable ASTM standards. How is this possible? This TrueFit poster from Charlotte Pipe will tell you.Charlotte Pipe & Foundry Co.