Andrew Lambirth relishes the reproductive originality behind seven decades of Parisian printmaking, as revealed at the British Museum
Why are prints so often thought of as the poor relations of the visual arts? Perhaps it's largely a matter of definition: what is a print as opposed to a reproduction? An original print seems like a contradiction in terms - how can something be both original and yet identically repeated? The original suggests the hands-on, individually created, while the printed suggests the mass-produced by machine. How are these opposites reconciled? Simply through the agency of the artist. The artist makes an image, and then it is produced as a print in a strictly controlled edition. Sometimes the edition is only one, which is called a monoprint; in other cases, perhaps 30 or 100 identical prints are made, numbered and signed, before the plate is destroyed. Prints are identified through this careful editioning, and their value is assessed by it. A print is not just a cheap photocopy of a precious original, but an image conceived as an etching or an engraving or a lithograph. It is very much a type of art in its own right.

At the British Museum, which has one of the best all-round print holdings in the world, there is currently an exhibition of considerable magnitude. Yet it has received very little coverage in the arts pages of the national press. Why? Probably because it's not thought of as an attractive (in terms of readership-seducing) subject. It's not sexy. But this is patent rubbish. Everyone who has actually seen the show is knocked out by it. It's very sexy indeed - in more ways than one.

For a start, it's full of the most ingenious and erotic nude images. There is, quite possibly, no greater erotic artist this century than Picasso. The show begins and ends with his work and runs the gamut of human sensibility in between. The very first exhibit is called The Frugal Repast and has all the dignity in suffering, the exquisite pathos, of Picasso's so-called "Blue Period" paintings. A couple, etiolated and breadstick-fingered, glance askance at a world that has traduced them. It's Picasso's first print made in Paris. He'd done one before in Barcelona, but had received no formal training in the medium. Technically, it is amazingly accomplished, with all the mastery of drawing, the authority, that we expect in his later work. Picasso himself was so impoverished at the time that he recycled the half-worked etching plate of a friend; although he scraped the metal down, you can still see traces of an earlier landscape by another hand in the top right corner. It's an image of great poignancy.

The date of The Frugal Repast is 1904, while the last image in the exhibition is another etching by Picasso made in 1971, just two years before his death. (It's a bravura brothel scene, all wiry line and lost causes, the old man lamenting what is no longer.) That is the time-scale of the show, and though containing the work of nearly 60 individuals, it is dominated by only three: Picasso, Matisse and Dubuffet. That said, there are literally dozens of little-known artists exhibited here, artists who are accorded a very well-deserved airing. The British Museum has actually acquired half of the prints on display in the last few years, representing a highly significant and entirely admirable development of its modern French print collection.

It is an extraordinary exhibition: there has never been a survey of this kind and depth before. Everything has come from the BM's permanent collection, apart from 16 items on loan from the Hunterian Art Gallery, part of the University of Glasgow, whither the exhibition will travel in reduced form next year. There are over 160 exhibits, but it's difficult to single out works for particular commendation - the quality of the show is consistently high. In the first half of this century, Paris was undoubtedly the centre of the art world, a magnet for international artists. This began to change in the 1960s, but, at least until then, the print workshops and dealer- publishers operated a large and successful trade in fine work. This exhibition concentrates on single-sheet prints, so none of the wealth of artists' books produced during this time is evidenced. None the less, it's a stupendous show.

All the main art movements that existed or were originated in Paris during this period are represented: Fauvism, Cubism, Neo-Classicism, Surrealism. It's a lasting irony that the Fauves, dubbed "wild beasts" because of their piquant colour combinations, should have favoured the black-and- white woodcut when it came to printmaking. The honourable exception in this show is Louis Valtat, whose two red woodcuts are ignited by touches of green and yellow. Matisse makes a splash throughout, principally on account of the boundless humanity of his images, and the rare quality of his drawing. "One must always search for the desire of the line," he said, "where it wishes to enter or where to die away. Also always be sure of its source; this must be done from the model." Matisse is Picasso's double: sounding-board and echo, contradiction and complement. He is superbly represented here by early works, mostly nudes, of great power.

The Cubists don't come off particularly well in print form, apart from a female figure of real sensuality by Josef Kapek (died Belsen 1945) and, of course, Picasso. Robert Delaunay, constantly undervalued, is represented by a curiously squashed aerial view of that great leitmotif of modernism, the Eiffel Tower. There's also a peculiarly potent colour woodcut by Jean- Emile Laboureur dealing with drug addiction: it depicts a woman in the throes of opium-induced lassitude, with a graphic skill and seediness that look like something out of Lautrec.

Aside from the renewed yet avant-garde classicism of Picasso and Matisse, what else was going on in the Thirties? One focus was SW Hayter's print workshop, Atelier 17. Hayter was a practical as well as practising Surrealist, who not only developed new methods of engraving and encouraged the tricks of automatism, but actively collaborated as a printmaker with some of the finest artists of his generation. Nearly everyone went to Atelier 17, and, when it moved briefly to New York during the Second World War, its sphere of influence increased. Hayter was a remarkable draughtsman himself, but was also able to facilitate the experiments of others. His example paved the way for the extraordinary freedom that artist-printmakers in Paris felt during this period. Anything was possible, and the so-called School of Paris generally attempted it.

We know that Picasso was constantly experimenting, at one point taking the lithographic ink off a plate with sandpaper, at another washing a plate in the shower, but he was by no means alone. Dubuffet inked up any texture that he liked the look of, and printed it. From 1958 to 1962 he made a whole series of prints called Phenomenes which captured the phenomena of the natural world, whether it was the earth we tread on, a bare human back or an old wall. These are images which appear abstract but which in fact could not be more directly representational. They are strangely beautiful imprints of the real world, as discovered by a great artist. Nine are on show, and for them alone this show is a must. From nearly all the other figures, particularly the lesser-known (Nono Reinhold? Sigismund Kolos? Kaiko Moti?), there are impressive and excellent things. Look, for instance, at the whited-out inkless intaglio print by the Romanian sculptor Etienne Hajdu - it's gorgeous. Prints have for too long been marginalised: do see this exhibition before it closesn

`Printmaking in Paris: Picasso and His Contemporaries' at the British Museum, London WC1 (0171-323 8525) to 14 Sept