A skylight, also referred to as a rooflight, is a window which is typically built into roofs. With their origins in Ancient Rome, skylights were once fully open structures bringing in both light and the elements. Nowadays, in modern buildings, they are commonly closed with glazing, featuring a frame which encases an openable sash attached by hinges. Similarly to fanlights, which refer to an overhead glass panel above doors, skylights optimise daylight, creating an elegant, spacious ambience. Varying in size from small slits to generous panels, skylights can brighten even the most difficult to reach, dimly lit of corridors while also providing a great supply of fresh air, ventilating rooms effectively when tilted open.