One piece elbows are fabricated from a a single piece of galvanized sheet metal. They are formed into an elbow by a special machine. Centerline radius is 2.5 times the elbow diameter. All elbows 3″ dia. to 8″ dia. are 22 gauge and 10″ dia are 20 gauge. Each elbow is welded air tight on the back. Long radius elbows have one male end and one female end.