Door Anatomy 101

A transom refers to the window that sits above the door. They are typically a half-circle, but can also be elliptical, rectangular, or come in a full circle. 

This is the structural post between the sideline (see next definition) and the door. Doors without sidelines do not have mull posts, they only have jambs. A mull post is essentially a “double-sided” jamb. This is used to help cover the seal between a door and the sidelines to give it a “cleaner” look and appearance. 

A fixed narrow panel, installed next to a door panel, for decorative purposes. They almost always contain glass lights. Sidelines 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/border around exterior doors. It is most commonly used in pre-hung units and will often be called trim or casing. Brickmold can be made of different materials: finger-joint pine, composite, vinyl, or stain grain wood. The key differences between materials are the cost and the durability (more durable = more expensive). 

The door slab is the largest and most important component, and probably the most well-known. It, however, is not the only one that matters. The performance of a door is ultimately tied to the components that surround it. The best door slab in the world will not perform with loose hinges, rotten jambs, or a leaky sill. 

A horizontal beam below the door that supports the frame; a flat metal bar screwed to the floor 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 the home sloping down to the outside. It always sends water away from the home/building. 

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 required part of the door frame, but a good investment for wet climates.