Many of the most famous old masters in our museums were once altarpieces, or parts of dismembered and partially destroyed altarpieces. The National Gallery owns the second version of Leonardo’s Virgin of the Rocks, along with two flanking panels painted by workshop assistants showing angels. The narrative relief sculptures that completely surrounded Leonardo’s panel were dismantled and lost before it was sold to Gavin Hamilton in 1785. The National Gallery’s Sainsbury Wing contains plenty of the subsidiary pictures that once accompanied the main panels of altarpieces, now neatly framed like easel paintings and easily turned into postcards. Three of the most famous come from the predella (or base) of Duccio’s massive double-sided Maestà (1303–11) in Siena Cathedral, which was dismantled, sawn in half and pruned in 1771. Similar panels are held by ten other museums. Most collectors, curators and connoisseurs had few regrets about these often brutal amputations. Once liberated from gloomy, crumbling churches in Italian backwaters, the fragments could be reborn in well-lit temples of High Art, shorn of papist superstition, ceremony, incense and song.

Art historians in the 19th and 20th centuries, with their interest in style and attribution, were rarely concerned with the meaning of altarpieces or with the evolution of the altarpiece as a religious object. Over the last thirty years, however, there has been a growing desire to reimagine original