Parts of a Door

  • Masonite Parts of a Door Diagram


A transom is a window above a door. Transoms in a half circle shape are also called fanlights (shaped like an open fan). There are different sizes and styles of transoms: rectangular, shoulder, half circle, full circle and elliptical.

Mull Post

Structural post between the sidelite and the door. Doors that do not have sidelites do not have mull posts, they simply have door jambs. A mull post in essentially a “double-sided” jamb. An installer could simply take two jambs and place them back to back to create a mull post. The only issue with “making” a mull post from jambs rather than buying one created for the purpose is that the “back-to-back” version will have a seam in the middle where the jambs meet and a one-piece mull post will not. A mull post has an interior and exterior mull cover to hide the seam.


A fixed narrow panel, installed next to a door panel, for decorative purposes. They almost always contain glass lights. Sidelights are normally designed to occupy a position adjacent to exterior doors, either in a common frame with the door or in a separate frame.


Brickmold is used as casing around exterior doors. It is molding around the window and door frames that abuts the exterior facing material of the building and serves as an aesthetic boundary between the siding and the frame; most commonly used in pre-hung units. It is also known as trim, casing and and an architrave. Brickmold can be made of different materials: fingerjoint pine, composite (e.g. extruded pvc), vinyl or stain grain wood. The key differences between material are the cost and the durability (more durable = more expensive).

Door Slab

While the door slab is the largest and most important component, it is not the only one that matters. The performance of a door slab is inextricably tied to the components that surround it. The best door slab in the world will not perform with sagging hinges, rotten jambs and a leaky sill.


A horizontal beam below the door that supports the frame; a flat metal bar screwed to the floor that prevents the door from swinging through and helps keep the elements out. Sills are designed with a slope/ramp that goes from the inside of home sloping down to outside. It always sends water away from the home/building.

Mull Boot

A plastic component that covers the end of the mull post (where it meets the sill); mull boots protect the mull post from water damage that leads to rotting. Not a requisite part of the door frame, but a good investment for wet climates.