what i think would be most important for you is you dont want to light of a arrow or a under-spine shaft for your bow example; say you have a 70 lb bow .never shoot a wood arrow in it. or a light under spine arrow.. when you release it the force of the energy from the bow will snap the arrow and the shaft might stick in your arm and come out your finger.the term grain is a measurement term also used on arrows to determine the weight of a shaft... the more weight the more kinetic energy you get, it also helps the bow absorb the energy making the bow more quiet.. but you loose speed, but you gain ke....we use the grains per inch to calculate the weight to get what we want.say i need a 450 grain arrow, at 28 inches, the shaft needs cut to get there so take 9.0 grains per inch off and you have...whatever..+fetching+tip+ inserts+nock+ insert=450 grains.. read up on kinetic energy, foc.spine, archers paradox. this will help you a lot.welcome to AT new guy...mike