Customize your treatment mechanics by achieving near constant spring forces to move teeth to a more predictable end result.  Constant Force Springs can be stretched up to 3 x their passive length to achieve the desired result.Two Spring Lengths 9mm & 12mm.Five Force Levels - Extra Light; Light; Medium: Heavy; Extra Heavy.Constant Force during unloading.Force ID (Located on Eyelet):1 Hole - Extra Light.2 Holes - Light.3 Holes - Medium.4 Holes Heavy.5 Holes - Extra Heavy.Pack of 5.