Couplings slip inside the ends of two round ducts being joined together and then sheet metal screws are fastened to hold it in position. Couplings may be demanded at a specific size like connections 18” and larger. The shaded area inside the impression underneath could be the coupling. They are https://round-conduit13445.onzeblog.com/36625075/new-step-by-step-map-for-round-conduit