The developed system of following a single senior marja al-taqlid is a comparatively modern feature of Twelver Shia practice, consolidating over the nineteenth century as the Usuli school, which holds that ordinary believers must follow the rulings of a living, qualified jurist, prevailed over the rival Akhbari school, which rejected reliance on independent juristic reasoning in favour of relying directly on the transmitted sayings of the Imams. The title ayatollah itself, meaning sign of God, only came into common use as a distinguishing honorific in roughly the same period, and grand ayatollah later still.
The office today spans a real division over what its authority is for. Iran's constitution, built on Khomeini's doctrine of wilayat al-faqih, vests supreme political authority in a ranking jurist, currently exercised by Ali Khamenei. The most prominent alternative, the quietist position associated above all with Ali al-Sistani, the senior marja based in Najaf, Iraq, holds that the clergy's proper role is religious guidance and that direct political rule falls outside a jurist's mandate, and Sistani has repeatedly declined the kind of governing authority the Iranian model assigns to its supreme leader. The two positions coexist within the same clerical rank without either having authority to bind the other, since, as in the office generally, no body sits above either man to settle the disagreement.