During the thirteenth century this style of design developed into what is known as the Geometric style. In this style, the windows became larger with the increased use of mullions, and the decorations became more complex and geometric in design.
In the decorated designs, the windows are subdivided by vertical stone bars known as mullions. The mullions are spaced close together usually rising the whole height of the window up to the springing line. The springing line of a window or arch is the line at which the arch shape starts from. The area above the springing line of the window is full of complex shapes of stone called tracery. With higher walls and larger windows, the cathedrals were now becoming more open to the light. The majority of Exeter Cathedral is built in this style.