A collection of beautifully-crafted grilles at a monastic ruin has puzzled scholars for years.

They have examined every nook and cranny of 850-year-old Rievaulx Abbey, near Helmsley, North Yorkshire, in a bid to reveal the original purpose of the grilles.

English Heritage is to display the grilles, some of which experts now believe were inset in the painted glass windows in Rievaulx's church, at the abbey's museum.

Senior curator at the site Andrew Morrison said: "We have two million relics in our North Yorkshire store alone, and many defy attempts to explain their original purpose.

"Even so, we are now fairly certain the grilles were used to ventilate spaces such as Rievaulx's huge abbey church.

"The late medieval windows were destroyed at the time of the suppression by Henry VIII and their true appearance was previously unknown.

"Therefore, these previously-overlooked grilles may hold the key to the windows' true design."

The need for sophisticated ventilation systems at Rievalux stemmed less from reasons of personal hygiene as the need to expel fumes from the burning of incense and tallow candles.

Research was aided by volunteer Jacquelyn Frith, of Nunnington, North Yorkshire, who has studied lead collections for English Heritage.