A girder bridge is perhaps the most common and most basic bridge. The cross section of the girder takes the shape of the capital letter "I". The vertical plate in the middle is known as the web, and the top and bottom plates are referred to as flanges. A box girder is much the same as an I-beam girder except it takes the shape of a box. The typical box girder has two webs and two flanges. However, in some cases there are more than two webs, creating a multiple chamber box girder. Other examples of simple girders include pi girders, named for their likeness to the mathematical symbol for pi, and T shaped girders.