Stained glass windows are one of the most distinctive aspects of Gothic architecture, which emerged in medieval Europe in the 12th century. Building technological advancements, such as tracery (a line of stone supports between glass parts), enabled larger and more complex windows. Glass was commonly used instead of lead for window frames because it did not conduct heat like metal does. This made glass an ideal material for allowing light into buildings while keeping out cold weather. Gothic windows typically feature large areas of clear glass with small colored squares or triangles inserted into the glass to create patterns. The colors used in stained glass come from minerals and other substances that contain those colors: red from iron oxide, green from copper oxide, yellow from gold, and blue from cobalt. Windows were often painted with multiple colors to produce tinted glass, but white glass could also be used to allow more light into dark rooms. During the Late Middle Ages and Early Modern Era (1450-1700), artists began to add detail to windows that hadn't been possible before. For example, they would paint flowers on window boxes or animals in landscapes. These paintings are called "cartoons" because they were originally drawn onto paper and then transferred into windows. Today, these cartoons are what give windows their unique appearance.