In the differential geometry of curves, an involute (also known as evolvent) is a curve obtained from another given curve by attaching an imaginary taut string to the given curve and tracing its free end as it is wound onto that given curve; or in reverse, unwound. It is a roulette wherein the rolling curve is a straight line containing the generating point. For example, an involute approximates the path followed by a tetherball as the connecting tether is wound around the center pole. If the center pole has a circular cross-section, then the curve is an involute of a circle.
Alternatively, another way to construct the involute of a curve is to replace the taut string by a line segment that is tangent to the curve on one end, while the other end traces out the involute. The length of the line segment is changed by an amount equal to the arc length traversed by the tangent point as it moves along the curve.
The notions of the involute and evolute of a curve were introduced by Christiaan Huygens in his work titled Horologium oscillatorium sive de motu pendulorum ad horologia aptato demonstrationes geometricae (1673).
Parametric curve 
Equations of an involute curve for a parametrically defined function ( x(t), y(t) ) are:
Involute of a circle 
The involute of a circle resembles an Archimedean spiral.
- In Cartesian coordinates the involute of a circle has the parametric equation:
where is the radius of the circle and is an angle parameter in radians () .
- In polar coordinates the involute of a circle has the parametric equation:
where is the radius of the circle and is a parameter in the interval
With a different parameter with it can be written in the form:
Curve length 
The arc length of the above curve for is
Involute of a catenary 
Where: t is a parameter and sech is the hyperbolic secant (1/cosh(t))
to get .
Involute of a cycloid 
Where t is the angle and r is the radius
The involute has some properties that makes it extremely important to the gear industry: If two intermeshed gears have teeth with the profile-shape of involutes (rather than, for example, a "classic" triangular shape), they form an involute gear system. Their relative rates of rotation are constant while the teeth are engaged, and also, the gears always make contact along a single steady line of force. With teeth of other shapes, the relative speeds and forces rise and fall as successive teeth engage, resulting in vibration, noise, and excessive wear. For this reason, nearly all modern gear teeth bear the involute shape.
The involute of a circle is also an important shape in gas compressing, as a scroll compressor can be built based on this shape. Scroll compressors make less sound than conventional compressors, and have proven to be quite efficient.
See also 
- John McCleary – Geometry from a Differentiable Viewpoint / Cambridge University Press, 1995 / pg. 73