Karl Feuerbach (1800-1834) was a geometer who discovered the nine-point circle of a triangle.

One of the most striking theorems in triangle geometry is Feuerbach's theorem. He wrote in his 1822 paper:

The circle which passes through the feet of the altitudes of a triangle touches all four of the circles which are tangent to the three sides of the triangle; it is internally tangent to the inscribed circle and externally tangent to each of the circles which touch the sides of the triangle externally.