Four-vertex theorem

An ellipse (red) and its evolute (blue), showing the four vertices of the curve, each vertex corresponding to a cusp on the evolute.

In geometry, the four-vertex theorem states that the curvature along a simple, closed, smooth plane curve has at least four local extrema (specifically, at least two local maxima and at least two local minima). The name of the theorem derives from the convention of calling an extreme point of the curvature function a vertex. This theorem has many generalizations, including a version for space curves where a vertex is defined as a point of vanishing torsion.