The “Sliding Cross” PVC fittings (aka pass through or slip cross) are commonly used in building PVC structures where you don’t want to compromise the strength of the pipe by cutting it. With no stops in one direction of the cross, you can easily slide it onto the pipe where you want it. These work well for the ridge of arched greenhouses because the pipe will pop out of normal cross when it is bent. If you want to fix them permanently after they are set in place, they can be glued or put a self tapping screw through the fitting into the pipe. All outlets are non-threaded sockets and are compatible with furniture grade PVC pipe or standard PVC pipe from a hardware store.