The marja'iyya is Shia Islam's answer to authority during the Imam's occultation: every observant Twelver selects a living grand ayatollah to emulate in matters of practice, and follows his rulings. It is a genuinely voluntary market for religious authority — believers may switch, and marja compete on scholarship and reputation — and it concentrates enormous influence, as Ali al-Sistani in Najaf demonstrates. It also generated the twentieth century's central Shia argument over whether clerics should govern directly, which Khomeini answered yes and Sistani, in practice, has not.