ART / Always check the small prints: Revised, copied and faked, original prints by Old Masters are difficult to spot. Which, says Dalya Alberge, is part of their appeal

There are Rembrandt prints and there are Rembrandt prints. The rarest were printed by the 17th-century master himself, but some were run off by his workshop, some by other printers. Centuries later others were run off from the original plates, still more were printed from plates made by copyists copying Rembrandt images. That's not to mention the photo-mechanical reproductions.

Confused? You're not alone. Prints, as any specialist in the field will acknowledge, are a tricky business. Not only are there many different techniques - etching and engraving, woodcut and drypoint, mezzotint and aquatint - but, to complicate matters, 'print' is an all-purpose word: it is used not only for an original work of art, printed in relatively small numbers by the artist himself, but also for posters, postcards and photographs, which can be knocked off with the ease of photocopying.

The chances of picking up an original off a market stall are remote. There are fewer than 50 reputable print-dealers worldwide and, at the annual London Original Print Fair at the Royal Academy, you will find the most respected of them.

As Elizabeth Harvey-Lee, one of this year's 22 exhibitors, says: 'A lot of Old Master prints are reasonable in comparison with modern prints - yet most people think the older ones are more expensive . . . You can buy a nice Old Master for a few hundred pounds.' That will buy you a print by Jan Lievens, who shared a studio with Rembrandt and whose work is good enough to have been misattributed to him, or something by the great, though relatively neglected, French etcher Jacques Callot (1592-1635).

Today, a photograph can fetch more than a print made centuries ago. Yet on a stand taken by Olimpia Theodoli, an Italian-born dealer, prints can be had for as little as a tenner. Their investment value is nil - they are often only one in a run of thousands - but their charm is in having been printed in the 16th century. At the other end of the scale, for pounds 10,000, she has a Rembrandt done by Rembrandt - his Nativity. Here, part of the right-hand corner is missing, whereas the second state indicates that Rembrandt later altered the plate. Rembrandt frequently altered his plates - printing some in four to six different states to correct errors in the print-run. An image that Rembrandt saw as flawed is, for today's collectors, all the more rare and valuable since an error often indicates an early state.

There is a limit to how many impressions a plate or block can produce: the earlier the impression, the clearer the lines. Copies are easier to spot. Theodoli has put two versions of Rembrandt's Rat Catcher on sale side by side - a pounds 3,000 second-state print, with a 17th-century artist's copy thrown in for free. The real give-away is that the copy is a reverse image of the original (see illustration). So even if you jib at the price, at least you can be sure of not falling for any false impressions.

Royal Academy, Piccadilly, London, W1, 14-17 Apr, 11am-6pm daily; pounds 5 incl catalogue ( pounds 2.50 concs)

(Photograph omitted)