Decorated architecture has window tracery. Elaborate windows are subdivided by closely-spaced parallel mullions (vertical bars of stone). The mullions then branch out and cross, intersecting to fill the top part of the window with a mesh of elaborate patterns called tracery. The style was geometrical (more straight lines) at first and flowing (curves) in the later period. This evolution of decorated tracery is often used to subdivide the period into an earlier "geometric" and later "curvilinear" period.