The Tiemco Twin Hackle Gauge is a must-have tool for making consistent flies. To set the gauge, just slide down the stem of the fly tying vise through the hole. In order to tie beautiful flies, the TMC Twin Hackle Gauge has two pins; one is for standard patterns, the other one is for parachute patterns. Expert fly tiers know that they need two different sizes of hackle. The hackle will not slip around the tapered pins while measuring. The hole is a little bigger than the diameter of vise stem so that the gauge will have an angle towards the user and rotate easily while not in use. Fits onto most vise stems.