The Terra Hackle Gauge is used to determine the proper sized hackle for the size of hook being used. This handy little gauge fits on most vise stems rotating freely so it can be swung out of the way when not in use. To determine the 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 graph the correct size. When tying dry flies it is important to use the proper sized hackle so the fly will float correctly.