In the warm Mediterranean and Middle Eastern climates, the courtyard served to hold the large number of worshippers during Friday prayers. Often, hypostyle mosques have outer arcades. They allow the visitors to enjoy the shade. Arab-plan mosques were built mostly during the Umayyad and Abbasid dynasties. The Arab plan was very simple, which did not allow for much further development. This caused that style of mosque to fall out of favour. The flat roof of the prayer hall was supported by columns. Many rows of columns were needed to support such roofs;. this is called "hypostyle architecture". Today it houses the main church of the diocese of Cordoba in Spain. One of the most famous hypostyle mosques, it is supported by over 850 columns.