A pinstem is a jewelry finding affixed to the reverse of a brooch, pin or tie tack specifically designed to pierce fabric for the purpose of fastening the object to an article of clothing. The hinged pinstem on a brooch is usually held by a clasp or catch, ranging in design from a simple "C" to one with an intricate safety mechanism.

Pinstem on Spider Brooch.jpg

Reverse View of Antique Amethyst and Garnet Spider Brooch Featuring the Pinstem.
Photo Courtesy of Lang Antiques.