By 1905 they were a fixture: every Wednesday, composers and singers crowding into the drawing room of apartment 39, no formal program, just whoever showed up playing whatever they had been working on. Glazunov premiered pieces here. So did Lyadov, Taneyev, Scriabin, Rachmaninoff. The young Stravinsky - then in his early twenties, still a year or two from The Firebird - played his early experiments under the eye of his teacher. The bass Feodor Chaliapin sang. Nadezhda Zabela-Vrubel, wife of the painter Mikhail Vrubel, accompanied him. The Becker piano Rimsky-Korsakov bought in 1902 still stands in the same corner of the same room. The gatherings, miraculously, still happen - the museum hosts chamber concerts on the original instrument, in the original room, for audiences who sit roughly where Stravinsky once sat being terrified.

After the 1917 revolution, Rimsky-Korsakov's widow Nadezhda was forced out. She had time, before she went, to do one extraordinary thing: she catalogued everything. Her husband's manuscripts, the music library, the posters, the family photographs, the Becker piano, the writing desks, the corner stoves, the wallpaper - all of it documented, much of it removed for safekeeping. The apartment then became a kommunalka, a Soviet communal apartment, where ten families and twenty-some people lived among Rimsky-Korsakov's furniture for half a century. Three generations of his descendants lobbied for the apartment to be made a museum. The decision finally came in 1967. They peeled back layers of paint to find the original wallpaper underneath. Over 250 of Nadezhda's catalogued items came home, returned to the rooms where they had stood when her husband worked.

Rimsky-Korsakov was the youngest of the Mighty Handful - the Moguchaya Kuchka, the five composers (with Mussorgsky, Borodin, Cui, and Balakirev) who set out in the 1860s to build a uniquely Russian classical tradition free of German influence. By the time he moved into Zagorodny Prospect in 1893, he was the last one still composing seriously. Mussorgsky was fifteen years dead, Borodin twelve. Rimsky-Korsakov spent his last fifteen years here finishing what the others had left undone - editing Mussorgsky's Boris Godunov into the version the world would hear, finishing Borodin's Prince Igor with Glazunov, and writing his own late masterworks. Eleven of his fifteen operas were composed in this apartment. The Tale of Tsar Saltan, Sadko, The Legend of the Invisible City of Kitezh, the political satire of The Golden Cockerel that nearly got him in trouble with the censors - all of them began at the desk in the silent study, with Nadezhda at her own desk nearby making the piano arrangements as he wrote.

Among the museum's most precious objects is a fountain pen given to the composer in 1880 to mark the premiere of his opera May Night. He used it for the next twenty-eight years - to write every score, every letter, every entry of the autobiography he called My Musical Life. The pen sits in a glass case in the apartment now, alongside portraits of his ancestors. The Rimsky-Korsakovs were noble but not particularly grand: the family descended from a Lithuanian named Ventseslav Korsak who came to Russia in the 1400s. Tsar Fyodor III added the prefix Rimsky in 1677, on the basis that the family's distant ancestors had supposedly been Roman subjects. Nikolai Andreyevich was the fourteenth Korsakov in Russia. The Roman connection had grown thin by then. The music had not.

The museum sits in a courtyard that surprises you - you walk in off Zagorodny Prospect, past the modern shop fronts, and suddenly the city noise drops and you are in a quiet inner space ringed by greenery and old facades. The five-storey building is unremarkable, the staircase is plain, and then you are in apartment 39 standing in the dining room, looking at portraits of Rimsky-Korsakovs from the 18th century. The original tiled corner stoves are still here. The marble fireplace. The folding doors with their bronze handles. The writing desk where The Tsar's Bride was composed. Most music museums show you instruments their subjects never touched. This one shows you the room they actually worked in, almost exactly as they left it.