A collar pin (closely related to the collar bar and collar clip) is a piece of men's jewelry, which holds the two ends of a dress shirt collar together and passes underneath the knot of a necktie. Functioning in a similar way as a tabbed collar, it keeps the collar in place and lifts the knot to provide a more aesthetically pleasing arc to the necktie.
a collar bar or barbell whose ends screw off and is designed to pass through specially made eyelets in each side of the collar
a pin, similar to a safety pin, that pierces each side of the collar (or passes through the existing eyelet)
a bar with clips on both ends that grasp each side of the collar