The sahn is the open courtyard at the center of a hypostyle mosque, typically rectangular, paved in stone, and framed on some or all sides by a riwaq, a shaded colonnaded arcade that extends the covered prayer space around the courtyard's edge. At its center or near one wall stands a fountain, basin or well used for wudu, the ritual washing of hands, face, arms and feet required before prayer, and the sound and sight of running water at the courtyard's heart is one of the most consistent features of congregational mosque design across regions that otherwise differ enormously in style.
The courtyard also absorbs overflow congregation on Fridays and during Ramadan, when attendance regularly exceeds the capacity of the covered prayer hall, and its open sky provides ventilation and daylight to a building type that is otherwise, in its hypostyle form, a deep and largely enclosed hall of columns. In hot climates the sahn's paving and any surrounding trees or arcades moderate temperature for worshippers gathering before and after prayer, and the courtyard frequently doubles as a space for religious instruction and community gathering, functions that extend the mosque's role well beyond the moment of formal prayer itself.