The chancel is the space around the altar at the liturgical east end of a classic Christian church structure, containing the choir and the sanctuary (sometimes known as the presbytery). It might end in an apse. The word comes from the Latin caelum, meaning sky or heaven. In Christian churches today, the term "chancel" usually refers to the area behind the altar where priests celebrate the sacraments. In medieval and Renaissance churches, however, the word took on its present meaning of the entire space between the altar and entrance door, which was often covered by a screen or carved wood paneling. Until the late 14th century, when stained glass became available, all church windows were made of translucent materials such as oiled paper or silk and were used primarily for decoration. In the late 13th and early 14th centuries, some cathedrals began to install movable wooden panels that could be opened to allow fresh air into the nave and closed to prevent intruders entering the church during services. These opening panels are believed to have been a new innovation that resulted in changes to the interior design of European churches. They provide an example of how contemporary design principles can be applied to religious buildings. Panel architecture quickly became popular throughout Europe.