Sarah Churchill, Duchess of Marlborough wearing the symbol of her office and authority - the gold key, by Sir Godfrey Kneller (National Portrait gallery - London UK)
From From her Wikipedia article; lightness added by gogm to bring out details.
Sarah Churchill wears a plain white chemise under a plain white satin robe that has a belt to define a natural waistline (and hand the key) in this Kneller portrait. Her robe has a criss-cross section and Kneller shows the satin there is somewhat translucent.
Keywords: Kneller, British, Churchill family, Sarah Churchill, Duchess, straight coiffure, chemise, scoop neckline, bertha, elbow length close sleeves, robe, vee neckline, elbow length flared sleeves, belt, natural waistline, full skirt