A collar pin is a piece of mens jewelry used to bind two sides of a collar together; much like the top button of a dress shirt. In addition to binding the collar together, the collar pin also lifts the knot of the wearers tie up making it appear more prominent and imposing.
Collar pins operate similar to safety pins and in fact could be technically interchangeable if sized appropriately. Collar pins are only worn with dress shirts specifically designed to accommodate them. These shirts will not have a top button, however will have a small embroidered hole on each side of the collar to accommodate the pin without requiring a piercing of the fabric.
Today collar pins are much less common but can make an excellent accessory for a necktie. Please note that neckties should never be worn with collar pin as well as tie pin, tie bar, or tie chain. Only one of these additions should be used at a time (if at all).