double Thinning Shears are shears with teeth on both sides instead of having one straight blade. If they are well made they can cut without leaving a visible line where they were closed. You can comb out with the blades closed in any direction. With a straight edge on one side, if you comb out so the hair is in contact with the straight edge, the hair can pull or be torn against the straight edge. With teeth on both sides you can turn the shear in any direction on the way out of the section without concern