If I may I will add to Dick's statement that the two beads may also be used to tell a shooter that the gun must be held off balance if it does not fit you correctly. Perhaps requiring more or less cast of or on, a higher or lower comb. Some guns don't shoot to where those two beads align to. I don't think thats their original purpose, but they help in this way. Trap guns with adjustable stocks can especially benefit from twin beads in this way.