A merlon is the solid upright section of a battlement or crenellated parapet in medieval architecture or fortifications. Merlons are sometimes pierced by narrow, vertical embrasures or slits designed for observation and fire. The space between two merlons is called a crenel, and a succession of merlons and crenels is a crenellation. Crenels designed in later eras, for use by cannons, were also called embrasures.