[url=https://en.wikipedia.org/wiki/Evolute]Evolute[/url] is the envelope of the normals to a curve. Equivalently evolute of a curve is the locus of all its [url=https://en.wikipedia.org/wiki/Osculating_circle]centers of curvature[/url].

Curve [i]e [/i]is [url=https://en.wikipedia.org/wiki/Involute]evolvent[/url] (=involute) of an circle [i]c[/i]. Determine a normal line to evolvent [i]e[/i] at moving point [color=#3d85c6][i]A[/i][/color]. Swich the trace on and determine the envelope of all normals.[br][b]Theorem:[/b][br][i]A curve is the evolute of any of its evolvents.[/i]