Used in combination with a straight length, this component assures that an exact required vent length can be achieved. Allows for easy disassembly and can absorb the linear expansion of longer pipe lengths. 12" length adjusts from 3-1/2" to 10-1/2", 18" length adjusts from 6" to 16". Size refers to the inside diameter of the pipe or fitting. The outside diameter of RV is approximately 1/2" larger. Type B gas vent. Can be used as part of a water heater vent pipe.