If you've ever tried to do the axis levelling on a compound bow, you'll know the first step is to confirm your string is perfectly vertical in the bow-jig. This tool will do that.

Simply clamp it to your string, and wiggle the bow in the jig until the bubbles are both level. Your string is now perfectly vertical and you can go on to do the 1st axis levelling of your sight.

You'll also need two 8mmx35mm bubble levels and an M6 nut and a 30mm M6 bolt. The bolt I used has a wingnut top.