In 1933, the Philadelphia Museum of Art acquired a painting that previously had been believed to have been painted by Johannes Vermeer. There was one, major problem: The discovery of a nearly identical painting in London six years earlier had cast doubt on that belief.

Art historians have spent nearly a century analyzing the two paintings, determining the London painting, "The Guitar Player," to be the original. But the artist behind the Philadelphia version, "Lady with a Guitar," and the reason for its existence remain a mystery. MORE: 'Task' Episode 2 recap: As stakes climb, HBO's Delco crime drama digs deeper into backstories

For the first time, the paintings are being displayed side-by-side at London's Kenwood House as part of the exhibit Double Vision: Vermeer at Kenwood , wh

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