The small trim ring, traditionally glossy black plastic, that dresses the joint where the shaft enters the hosel on irons and woods. It is decoration, not engineering: the ferrule smooths the visual step from hosel diameter down to shaft diameter and has no effect on how the club performs — Ping famously omits them altogether. Club builders like them anyway, since a ferrule tidies the epoxy joint during assembly and can be turned down flush with the hosel for a seamless look. A ferrule that pops loose and rides up the shaft looks alarming, but it's a five-minute cosmetic fix, not a broken club.