Used when an offset is required to avoid obstructions such as rafters, joists and other construction obstacles. Used to orient pipe between vertical and horizontal orientation. Four piece adjustable elbow twists for easy offsets. Size refers to the inside diameter of the pipe or fitting. The outside diameter of RV is approximately 1/2" larger. 3" elbow - 3 gore; 4", 5", and 6" elbow - 4 gore. Type B gas vent.