Rain cap is necessary: Avoid the mount threads exposed to accumulate dirt and to corrode Radio antenna base nut: Effectively protects the unused NMO antenna in all environments O ring Seal in the Base: keep water out. And there is an inner ring to protect the contact pin Waterproof: The rain cap the best way to keep your NMO mount dry when washing your car Compact Design: Very handy to use, perfect snug and sturdy Package include: 2 pc antenna caps.