Her Wikipedia article is here.
From Wikipedia.
Lady Catherine wears an English hood with a striped coif beneath it. The wide veil hangs down from the sides. Her dress has a blackwork collar and vest-like upper bodice. Holbein hints at the use of pearls in Catherine's hood.
Keywords: 1534, Holbein, Duchess Katherine of Suffolk, Willoughby family, Bertie family, Duchess, British, English hood, jeweled headdress, veil, high enclosing neckline, blackwork, full sleeves, split over-bodice