In the early 19th century, Poncelet, Feuerbach and others showed that in any triangle, the following nine points are cyclic:the midpoint of each side of the triangle, the foot of each altitude, the midpoint of the interval joining each vertex of the triangle to the orthocentre.
That is, these nine points lie on a circle.
He also showed that the centre of this nine-point circle lies on the Euler line, and is the midpoint of the interval joining the circumcentre to the orthocentre.