Denote the center of Curvature at by.When the point moves along the plane curve, the trace curve of the point is called the evolute of the curve, denoted by, and is called the involute of the curve.

• 当点沿曲线移动时，相应的曲率中心的轨迹曲线称为曲线的渐屈线，而曲线称为曲线的渐伸线。