On the ground floor aisles were placed to the left and right of the nave, and were open to pedestrian traffic, hence they were called ambulatories. Above each ambulatory was a triforium, a series of arches lit by windows kept out of sight from below. At the top was the clerestory, a series of windows that allowed light to come streaming through. Shortly after construction, the builders decided that the unfiltered light was too bright for the inside of a church, and the glass was stained so that the church was filled with various colors. Hence a new art form, the art of stained glass windows, was born.
