In architecture, structural engineering or building, a purlin (or historically purlin, purloyne, purling, perling) is any longitudinal, horizontal, structural member in a roof, except a type of framing with what is called a crown plate. In traditional Timber framing there are three basic types of purlin: purlin plate, principal purlin and common purlin. It is the practice in the steel industry that structural shapes are assigned representative designations for convenient shorthand description on drawings and documentation: Channel sections, with or without flange stiffeners, are usually referenced as C shapes; Channel sections without flange stiffeners are also referenced as U shapes; Point symmetric sections that are shaped similar to the letter Z are referenced as Z shapes. Section designations can be regional and even specific to a manufacturer. In steel building construction, secondary members such as purlins (roof) and girts (wall) are frequently cold-formed steel C, Z or U sections, (or mill rolled) C-sections. Cold-formed members can be efficient on a weight basis relative to mill rolled sections for secondary member applications. Additionally, Z sections can be nested for transportation bundling and, on the building, lapped at the supports to develop a structurally efficient continuous beam across multiple supports.