It has been asserted that Leonardo "almost invariably commenced two versions of each of his works, which he rarely finished". A number of experts have argued that Leonardo made two versions of the "Mona Lisa" (because of the uncertainty concerning its dating and commissioner, as well as its fate following Leonardo's death in 1519, and the difference of details in Raphael's sketch—which may reflect that he made the sketch from memory). Experts universally agree that Raphael's pen-and-ink sketch, in which the columns flanking the subject are more apparent, is based on Leonardo's portrait. Other later copies of the "Mona Lisa", such as those in the National Museum of Art, Architecture and Design and The Walters Art Museum, also display large flanking columns. As a result, it was thought that the "Mona Lisa" had been trimmed. However, by 1993, Frank Zöllner observed that the painting surface had never been trimmed; this was confirmed through a series of tests in 2004. In view of this, Vincent Delieuvin, curator of 16th-century Italian painting at the Louvre, states that the sketch and these other copies must have been inspired by another version, while Zöllner states that the sketch may be after another Leonardo portrait of the same subject.