What term describes the members that support the main frame of a door horizontally?

The term that describes the members supporting the main frame of a door horizontally is "rails." Rails are the horizontal components of a door frame, positioned at the top and bottom, and sometimes in between, that provide stability and strength to the door structure. They work alongside the vertical members, known as stiles, to create a robust framework that is essential for the door's functionality and durability.

In the context of a door, the rails are crucial as they hold the panels together and can affect the overall appearance and performance of the door. Knowing this distinction is important for anyone involved in construction or carpentry, as it directly relates to the integrity and design of door assemblies.