The mosque is not essential for worship, but provides space for meetings, study and worship, and Muslims gather there to pray, especially on Fridays. The principal officials are: the muezzin, who calls the faithful to prayer, the imam who leads the prayer, and the preacher (Khatib). It is important to be correctly dressed for prayer. Men should cover their bodies from navel to knees; women may show only their hands, face and feet. Clothing for both sexes should be plain. Traditionally, men and women pray separately, and special areas in the mosques, usually upstairs are reserved for women, although women generally pray at home. Because Muslim prayer involves standing, kneeling and prostration, an open space is provided, without chairs or benches, and a large space is required for the crowds who attend on Fridays. The direction of Mecca (qibla) towards which Muslims face when they pray is indicated by the mihrab. The raised pulpit (minbar), which the preacher mounts to deliver his Friday sermon (khutba), is the most important piece of furniture in the mosque. Every mosque has at least one minaret; though some have two or four and each minaret has one or more balconies from which the muezzin utters his call to prayer, five times daily. The mosque has no altar. For many Muslims the dome is the symbol of the oneness of God. Some mosques are decorated with beautiful wall-tiles, often bearing calligraphic designs based on verses from the Koran.
Traditionally, images are not used, and in place of these, miniatures, flower-and-leaf designs and calligraphy were developed. In the Koran as in the Torah there is no verse which clearly forbids representative art. Later in the Hadith, depiction is clearly forbidden. To make representations of Nature is considered to be a turning aside from the truth. The Hadith says that those who depict living beings are presuming to imitate God’s creation and will be punished in the afterlife. Embellishment of the Koran, which sometimes takes years to complete, is widespread. Abstract and geometric designs and many different calligraphic styles of Arabic script are used as decoration, regarded as appropriate to adorn the Word of God. From the 10th century onwards, special academies or seminaries were established to teach religious and legal doctrines.