A stained-glass window is composed of many small pieces of glass, held in place by narrow bars of lead. Lead can easily be shaped into any form, as we know it from the colourful glass mosaics in the Romanesque and Gothic churches. For constructive reasons leadlights are reinforced with iron rods on the outside of the window.  

The oldest examples of windows with stained glass can be found in the cathedral of Augsburg in Bavaria, built in the beginning of the 12th century.