What is a transom? A small window that is above a door or a larger window. In Britain it is referred to as a fanlight. In France it is referred to as a "Vasistas". The original purpose of the transom window was to provide adequate air flow. The window would be able to be opened with a long metal rod. As times changes and air conditioning was introduced, transom windows now serve as more of a decorative element. It also serves as a nice way to have natural light in your vestibule or foyer of your brownstone.