The most valuable part of a picture can be its frame. According to Mr Alan Montgomery, head of Bonhams frames department in London: "It's the case quite often that a picture comes in and the frame is actually worth more than the picture that's in it.

"If people have a nice 18th century painting, they want to put it in a nice 18th century frame. It certainly improves the value. It's a growing market. We're always finding people with huge collections gathering dust that they don't realise have a market value.

Carved wooden 17th and 18th century frames tend to be the most valuable but plaster - "or composition as we call it" - frames from the end of the 18th century through the 19th and 20th centuries can also have value.

To the person in the street, carved wooden and plaster frames can look similar, both tending to have gilding - the gold colour laid over the surface. "Quite often, it's difficult to tell the difference. The easiest way to tell is if there's a chip out of it. If it's white underneath it's obviously made of plaster and they're much more fragile - the plaster ones are more likely to be damaged. And obviously with the carved wood ones, you can see all the decoration has just been carved away."

Condition is important for values. "Plaster frames often come with big chunks out of them. The gilding is important. In the 19th century, a lot of older frames were given a heavy gilding or painted over the top and that will "negatively affect the value."

Another important factor that affects values is if the picture has been reframed and if the frame has been cut down to size. "That really affects the value. We like to see frames with the original joints in the corners," he says.

Italian and French frames tend to be worth lots of money, but a specialist is usually required to examine the joints to establish their origin.

Frames from the 19th and 20th centuries make from £30 (€49) up to £1,500 sterling at Bonhams. A nice Victorian frame in good condition would be expected to make £200 to £300. However, with good quality 17th and 18th century frames "we're talking up to £7,000 or £8,000 and more for a single frame. Our record I think is £26,000 for a single frame," he says.

Standard painting sizes are more commercial, with huge or odd-shaped frames more difficult to sell "because nobody will have a painting to fit it. Perfectly square frames are very difficult to sell because there are very few perfectly square paintings. So that has an effect on the price."

Bonhams next general frames auction is April 5th and it has a fine frames sale on April 19th. Items include a French Barbizon - a group of 19th century artists - frame estimated at £400-£600. A rare 20-inch square Spanish 17th century frame with scallop shells and leaves carved all over it is estimated at £1,000 to £1,500.

According to Mr James O'Halloran, managing director of James Adam in Dublin: "There have been some occasions where we have valued something for let's say £5,000 and the picture itself is only of marginal interest, because the frame is a cracking Georgian frame, giltwood and in good condition. But generally speaking you might only come across that situation once in a blue moon."

Bonhams frame department. Telephone: 0044 171 393 3988