Some of my bows have two and some have one. If you shoot different width arrows and nock below a nocking point, the fatter shaft will effectively have a lower nocking point. If you nock the arrow above the nocking point, then if you shoot a skinny arrow and then a fatter wood arrow, the nocking point will remain the same. Now if you mix in some tapered shafts, they may need a slightly higher nocking point. Thankfully for me, it does not make that much, if any, difference when I nock the arrow above when going from 1918s to tapered 11/32" arrows. Hill nocked above, it did not seem to affect his accuracy much. I could see if the string was smaller than the throat of the nock, things could slip around, like a skinny string and a snap on nock with a large round hole in the throat of the slot. It is very irritating when my arrow keeps sliding up the string when deer hunting, I always end up adding a small tie on and just stick with one arrow out of that particular bow. I can feel the bottom nocking point with my thumb and nock without looking. The justification for one, given by Schulz about sliding the arrow down the string, has always been a mute one for me. With my heavy B55 strings, the nocks were just that little bit too tight to slide easily, although once shot in, the string had a shallow spot so the nocks would easily shoot without impinging the arrow flight like a tight nock will.