This Imperial Stack Duct line is used for distribution of supply and return air to branch lines throughout a forced-air system. The 3 1/4-in by 24-in duct is assembled by snapping together the 2 "L" sections using Drive and "S" cleats (sold separately) to form one rectangular section. This stack duct is made from 30-gauge galvanized steel for rust resistance and long-lasting durability. Its design is suitable for use in most heating, venting and air conditioning applications.