Belvedere audio guide,
five stops, free.

It is a square, one hundred and eighty centimetres each way, which puts the figures at close to life size. Stand near enough and the surface stops being flat. It glitters unevenly, in patches, rather than shining evenly.

That is not an accident and it is not all gold. Klimt drew the composition on the primed canvas in graphite, fixed it with a thin glaze, then covered the canvas in Schlagmetall, paper-thin brass leaf, everywhere except the skin, the hair, the coloured flowers on the woman's dress and the meadow. On top of the brass he set individual leaves of real gold and real platinum. Then he glazed the whole metal surface with a dark brown oil resin, and while that glaze was still wet he scattered countless tiny flakes of gold, platinum and brass into it. The Belvedere's conservators compare that last step to the Japanese lacquer technique called maki-e, where an artisan taps gold flakes out of a bamboo tube onto a wet surface and lets them sink in.

Now how it got onto this wall. Klimt showed it in June 1908 at the Kunstschau in Vienna, and called it Liebespaar, the lovers. It was not finished; a photograph taken during that exhibition shows bare patches in the lower left corner. Ten days after the show opened the committee of the Moderne Galerie decided to buy it, hurried along by the German section of the Moderne Galerie in Prague, which had declared an interest at almost the same moment. The price was twenty-five thousand crowns, the most the gallery had ever paid for a painting by a living artist. Less than twice that sum would have bought a villa in the Salzkammergut with the furniture in it.

Someone in the ministry wrote a note into the purchase file: the lower left corner of the painting by Klimt is not yet quite finished. Klimt, on holiday at the Attersee, wrote back on the sixteenth of July 1908 promising to complete it the moment the exhibition closed. He took the canvas home in November, worked on it for months, and the museum took delivery on the twenty-second of July 1909, still under the title Liebespaar. The name The Kiss turns up first in a museum catalogue in the autumn of 1909, after the picture was already the museum's.

While he had it back, he changed things. X-rays and infrared show that the man once had a beard, that his cloak was ornamented differently, and that the woman's left forearm was hidden under it. And he lengthened her lower legs by several centimetres. Look at her feet now. They push out past the wavy edge of the gold, over ground he had already laid with brass leaf and metal flakes, and he had to carry the meadow further right to follow them.

Who are they? Nobody knows. There is a brooch in a private collection holding a scrap of parchment Klimt painted with the same embrace, inscribed with the name Emilie, her monogram and his, and the date the thirtieth of August 1908 — Emilie Flöge's thirty-fourth birthday, spent with him at the Attersee while this canvas hung in the Kunstschau. An argument, not a fact, and the best one there is.

The painting was evacuated outside Vienna during the Second World War and has had its permanent place here since the museum reopened in 1954. It went to New York in 1986, Japan in 1989, Zürich in 1992, and Rome in 1997 for exactly one week. That was the last time it left the city.

A hundred and five centimetres high and two hundred wide, so it hangs like a window, and the view through it is a real place: the Alp Tussagn, east of Savognin in Graubünden, with Piz Toissa and Piz Curvér on the skyline. Segantini lived down in that valley and painted out of doors in it.

Then the tree. A birch leans out of the snow, and a woman hangs in front of it with her eyes closed, her red-blonde hair caught in the branches. Look at what is at her breast. It is a child's head, and it is growing out of a branch that twists like an umbilical cord. Now follow the big bough and the curve of her body: between them they draw a heart.

The Belvedere does not soften what this is. Segantini is showing the fate of women who live out physical desire and refuse motherhood. He called what they suffer for it the scourges of purgatory. Nothing is left to them but to accept what he thought they were made for; only then are they released. Out to the left, in the distance, are the ones further along: a child pushing up through the crust of ice towards its mother, two women who have got free of their trees, and a horizon that warms towards gold. That is the Nirvana of the story.

Because there is a story. Segantini's friend Luigi Illica, who wrote the words of La bohème, Tosca and Madama Butterfly, put into Italian a poem called Nirvana and presented it as a translation of a medieval Indian text. Segantini painted the woman in the tree four times: The Fruit of Love, The Punishment of Lust, this one, and The Angel of Life. They were his first Symbolist pictures.

Some of it is his own life. His mother died when he was seven. He ran away, was found on the streets of Milan and put into a reformatory, where a chaplain noticed that he could draw. He could barely read or write until his mid-thirties. And he was stateless all his life, so he could not get a passport and never once walked into his own exhibitions abroad.

He died on the twenty-eighth of September 1899, of peritonitis, high in the mountains, at work on the third panel of a triptych. Two years later Vienna held the biggest Segantini exhibition there had ever been, and that same year the Secession gave this canvas, which it had bought, to the state gallery. The museum's records first show it hanging in the Moderne Galerie in Vienna in May 1903. Before you go, find the signature in the lower right corner: G. Segantini, 1894.

Three figures stacked in a pyramid in the dark, close together and, as the Belvedere's own description has it, reading as isolated from one another anyway. The man at the top is Schiele again, crouched behind the woman with his knees up around her, bony where she is soft. She sits between his legs, looking down and away, absorbed in herself. At the bottom, half hidden, a child peers out from between her legs.

Schiele did not call this The Family either. He called it Kauerndes Menschenpaar, the squatting couple, and the Belvedere still lists it under that name with The Family in brackets. It got the title we use only after his death, when the critic Berta Zuckerkandl used it for the first time, and the reason that retitling is unbearable is the thing to know before you leave this room.

He showed the painting in 1918, at the forty-ninth exhibition of the Vienna Secession. He designed the poster for it. He had nineteen paintings and twenty-four drawings on the walls. Gustav Klimt had died that February, and at twenty-eight Schiele was the leading painter in Vienna. Edith was expecting their first child.

On the twenty-eighth of October 1918 Edith Schiele died of the Spanish influenza, six months pregnant. The child did not survive. Schiele died of the same illness three days later, on the thirty-first. The family in the painting never existed and never would have.

And yet it is not a portrait of them. The model for the woman is not Edith. It may well be Wally Neuzil, the woman from the painting you have just left. Look at the child now: Schiele painted it in late, over a bouquet of flowers that had been lying between the woman's legs, and the model was his nephew Toni. So the picture is not a record of a family. It is an idea of one, assembled from people who were not that family, by a man who was about to lose the chance.

One last detail, and it is a hard one. Parts of the canvas are unfinished. Look at the man's left hand, and then at his face a few centimetres above it. The face is fully worked. The hand is barely there.

The painting went straight from Schiele's studio to the Viennese collector and painter Hans Böhler in 1918, stayed with him through his years in New York, and the Belvedere bought it from him in 1948.