The diameter of the objective will basically determine the height of the rings. The size of the rings are determined by the size of the tube, 1" tube takes 1" rings and 30mm tube takes 30mm rings. The higher you go with the rings, the more you have to put your check off the stock which results in begin less accurate. Just a rule of thumb. An objective 40mm or less can take a low, a 42-44mm will take a Medium and a 50mm will take a high.