Mary, Princess Royal and Princess of Orange Wearing a Fichu Held by a Large Brooch.
Painting by Bartholomeus van der Helst, c.1652.

Fichu Pin

A fichu, from the French for neckerchief, is a triangular scarf, often made of lace, worn about the shoulders and pinned in front. A fichu pin, or lace pin as they are sometimes referred to, is a small pin used to secure the two ends of the scarf in the front.