This is a replacement plumbing vent cap for your RV. In order for the waste water and sewage to flow properly, the holding tanks must be vented to the outside. The vent cap prevents blockage of the vent pipe. Constructed from a quality grade plastic, this cap is designed to fit securely around a 1 ½" pipe. The two-piece construction allows you to remove the top cap on the off chance you need to clear the vent pipe.