In the plane of any triangle ABC, let
D = the midpoint of side BC,
G = foot on BC of altitude from A,
J = midpoint of segment AX,
(X, the orthocenter, is where the altitudes AG, BH, CI meet.)
As you see in the sketch, a circle passes through all nine of the points D,E,F,G,H,I,J,K,L. It is the nine-point circle of triangle ABC, and its center, N, is the nine-point center.