Today I went looking for 22 1/2 degree elbows for 2 1/2 inch PVC drain pipe (schedule 40). I was told that such elbows would be hard to find and that fittings for 2 1/2 inch PVC are "pressure fittings". I'm curious why this convention arose. Apparently ordinary non-pressure fittings are readily available for 2 inch and 3 inch PVC drain pipe.