Because the Quran is held to be the literal speech of God in Arabic, the writing of Arabic became a devotional act, and calligraphy grew into the central sacred art of the Islamic world. Early Qurans were written in the monumental angular scripts loosely called Kufic, laid out in gold and brown on parchment. The tenth-century vizier Ibn Muqla is remembered for the proportioned script, a geometric discipline in which every letter is measured in rhombic dots against the height of the alif, and the later masters Ibn al-Bawwab and Yaqut al-Musta'simi refined the rounded hands, Naskh, Thuluth, Muhaqqaq and their kin, that became the classical Quranic scripts.
Persian culture added the hanging Nastaliq, and the Ottoman calligraphers brought the art to a final polish. Since figural imagery is generally avoided in mosque decoration, the written word fills the space where other traditions put icons: the name of God, the shahada and whole surahs run in bands around domes and mihrabs. A calligrapher traditionally trains for years under a master before receiving an ijaza, a licence to sign work, and the practice is understood as spiritual discipline, the beautification of the word of God being itself a form of worship.