ORIGINAL: GMMAT
Then how do you explain setting one up with a level and a square? If it's perpendicular/plumb to the string......it's Perpendicular/plumb to the ground. Ifyou're using a bubble level to set your arrow......what's it "level" to?
You do all that to set up "center shot". This means that the arrow is set up to be launched in the most efficient manner from the bow. Tuning the arrow and bow together refines this efficiency to the idiosyncrasies of the bow and arrow. In an idealized set up of center shot the tiller is even and the arrow is "centered" on the string.