This handy gauge fits most vise stems. To determine hackle size, simply bend the hackle around the pin at the bottom of the gauge. The hackle will flare out and the tips will indicate on the curved graph the correct size hackle. To determine hook size simply place the hook on the hooks pictured on the gauge. This gauge will rotate freely on the vise stem so it may be swung out of the way when not in use. It will correctly identify size 4-22 hackles and 4-16 hook sizes.