Twelver Shia practice requires a layperson to follow a living mujtahid, a marja al-taqlid, on matters of law, and specifically forbids following a dead one on new questions — so the office renews with each generation. The financial arrangement is the structurally important part: believers pay khums, a fifth of surplus income, directly to the marja's office rather than to a state, which funds seminaries, stipends, hospitals and charities and makes the institution genuinely independent of any government. Ali al-Sistani in Najaf is followed by tens of millions and has used that position with notable restraint — his 2014 fatwa calling for defence against ISIS mobilised tens of thousands, and his insistence on elections shaped post-2003 Iraq, while he has consistently refused to govern. Iran's constitution embodies the opposing answer, wilayat al-faqih, in which the senior jurist holds political sovereignty; Khomeini's formulation was contested by senior clerics at the time and the Najaf establishment has never fully accepted it. The disagreement between the quietist Najaf model and the Iranian state model is the central institutional argument in contemporary Shia Islam.