A Revit MEP system family that routes air between mechanical equipment and air terminals, sized and routed with fitting logic similar to pipe. Ducts connect through connectors on mechanical equipment and air terminals, and Revit auto-generates fittings (elbows, tees, transitions) as runs are drawn or resized. Duct systems carry their own sizing and flow calculations, and clash detection against structure and other MEP systems is one of the most common coordination tasks built around them.