Her maiden name was Thistlethwayte; her article in thepeerage is here.
From fleurdulys.tumblr.com/image/36017125677; spots removed mainly from background with Photoshop by gogm.
Anne Thistlethwayte poses in a rural setting wearing a dress suitable for the Assembly Room at Bath or some other grand location. Jewelry is limited to her sleeves. Her coiffure follows the towering French fashion.
Keywords: 1777, Gainsborough, Anne Thistlethwayte, Countess of Chesterfield, Thistlethwayte family, Stanhope family, Countess, Countess of Chesterfield, British, high straight coiffure, modesty piece, scoop décolletage, zone bodice, bows, elbow length tight sleeves, cuffs, full skirt, over-skirt, shoes, jeweled sleeves, wrap, fringe