If C is a curve and C' is its evolute, then C is called an involute of C' Any parallel curve to C is also an involute of C' Hence a curve has a unique evolute but infinitely many involutes Alternatively an involute can be thought of as any curve orthogonal to all the tangents to a given curve
(n) A curve defined as the spiral trace of a point on a flexible line unwinding from around a line, circle, or polygon The contacting surfaces between gear teeth are designed as involutes