Restrained flange couplers help connect flanged end pipes to plain end pipes. One end bolts to the flanged pipe and the other has bolts that lock down around the outside of the plain end pipe holding it in place (see figure below). One of the advantages to this kind of adapter is that it can connect a wide variety of pipe types where both pipes are the same material or different materials. You can mix pipe materials as long as the flanged end can bolt/fit onto the specified flanged pipe and the other receiving end is sized for the correct pipe material diameter. For example, it is quite common to use a pvc adapter to connect a plain end pvc pipe to a flanged pump elbow (often cast iron or other metals). The biggest advantage is often the flexibility of the connection. The restrained flange coupler offers a little movement for pipe alignments so small errors in alignment can be compensated for. This make pipe installation much easier especially in applications like wet well discharge piping where having a little “wiggle” room in to install the pipes (often above 10 ft in length) is almost necessary.