The most common types are solid-core wood (S.C.), hollow-core wood (H.C.), or hollow metal (H.M.). All of the doors can be furnished for paint finish; wood doors, when it is requested, may be furnished with any particular wood veneer available, at an additional cost. (One particular type of door that may come up periodically on proposed construction jobs is the class B door. This door of wood or metal, but usually metal, has a two-hour fire rating and is primarily located at stairwells or other enclosures requiring fire protection for at least two hours.)