Book excerpt: Momentous developments occurred in the field of Islamic art during the 11th and 12th centuries - developments that were to affect its aesthetic direction for centuries to come, but which sprang from deep within a political and religious clash at the heart of the Muslim world. Iran, Iraq and Syria were to see the flourishing of such devises as proportional calligraphy, vegetal and geometric arabesque and muqarnas (stalactite) vaulting, but these innovations were propagated in a highly confrontational atmosphere that pitched the traditional Sunnism of the Abbasid Caliphate against the heterodox Fatimids of Egypt.