A nice little tool to help you mark the position of your 3 arrows, at 120 degrees from each other. There is a nocking point within, so that your cock feather is perfectly 90 degrees from your string! Fits shafts up to 9mm diameter, and 4" feathers. To get it on with an existing cock feather, just bend the open part of it and slide it over! Works fine with me 4" x .5" vanes. Sharpie works perfectly. Sure beats the 20 some $$$ jigs online. Prints in less than an hour!