These are hose barb bulkhead fittings. If you are reading this, you probably know about bulkhead fittings..and how they work. Basically, they attach a hose to a water source.. like a tub. What I liked about these fitting are the silicone washers.. there are two.. One goes on the actual bulkhead part, and the other is in the hose barb part. To install these, you need to drill through the wall (side ) off the container,, it is about a 3/4″ opening. I was going to use these to make a canister filter, from a mason jar, but it turns out these are too large.. But.. I can think of a thousand other uses.. like for my drip irrigation system. A simple tub to collect some rain water, and a solar powered system to distribute the water to my garden. As I said, the hose barb part has three sizes, 9mm, 12mm and 16mm. One heads up.. the hose barb washer is like the washer in a garden hose, it kinda sits inside. While I was unpacking this, that washer fell right out.. I saw it on the floor, so just be a little careful when you set these up…. Overall, I was thrilled with these fittings.. especially the washers. So much you can do with them..