The title of Philip Wilson Steer's painting, Seated Nude: The Black Hat, raises an interesting question, namely whether you can really be unclothed if you have a stack of feathers on your head. Nudity is a slippery term, as Steer's picture suggests. A century after it was painted, his work has the power to embarrass. The truth is that Steer's nude isn't: she is naked. The difference between the two is the subject of the opening chapter of Sir Kenneth Clark's book The Nude, now the inspiration for a show of 20th-century British paintings at the Middlesbrough Institute of Modern Art.