What is commonly a part of a hinge door frame?

A hinge jam is an essential component of a hinge door frame as it provides the necessary support and alignment for the door to operate properly. The hinge jam is the vertical section of the door frame where the hinges are attached, allowing the door to pivot open and closed smoothly. It also ensures the door is securely held in place and contributes to the overall stability of the frame.

On the other hand, while a head casing is a part of a door frame, it is not specific to the functionality of the door itself but rather serves as a decorative trim at the top of the frame. A window sill, found in window installations, does not relate directly to door frames. A floor joist is a structural element that supports floors and is unrelated to the door frame components. Thus, the hinge jam's role as a supportive and functional aspect makes it a key part of the hinge door frame.