We investigate the intermediate logic RN of the Rieger-Nishimura ladder — the 1-universal model of intuitionistic propositional calculus. The Heyting algebra dual to the Rieger-Nishimura ladder is the 1-generated free Heyting algebra. Is is easy to show that RN is the greatest 1-conservative extension of intuitionistic propositional calculus IPC. The logic RN was first studied by Kuznetsov and Gerciu, Gerciu, and independently by Kracht, whose papers claim interesting results but contain at best sketch proofs, some of which have serious flaws. We provide a systematic study of RN and its extensions. Joint work with Guram and Nick Bezhanishvili.