Bay windows feature prominently in Victorian architecture. They became popular in the 1870s. They are basically comprised of three windows, one that is parallel with the front of the building, and then two on either side set at an angle to meet the center window.
Bay windows can sometimes be square or polygonal in shape. The angles that are used in designing a bay window are 90 degrees, 135 degrees and 150 degrees. These parameters are used to determine the way the corners look in the window.
Bay windows almost look like a small addition to a room, giving the appearance of a bigger room by creating the illusion of more floor space. Bay windows from the outside, appear to overhang the side of the building.
Bay windows are usually only one story high, although they are sometimes two stories, extending the full height of a building. You will find a lot of homes in the San Fransisco area with bay windows, in particular on the Victorian homes in the area.
Bay windows look as though they are an alcove addition that has been added to a home or building, with little or no support.
An unusual feature of bay windows is that many are not in proportion to the building, and can appear "paunch-like", or like a fat tummy.