A set of four points, one of which is the orthocenter of the other three. In an orthocentric system, each point is the orthocenter
of the triangle of the other three, as illustrated above
(Coxeter and Greitzer 1967, p. 39). The incenter
and excenters of a triangle
are an orthocentric system.

The centers of the circumcircles of the points in an orthocentric system form another orthocentric system congruent to the first, and
are the reflection of the original points in their common nine-point
center (Wells 1991).

The centroids of the points in an orthocentric system form another orthocentric system similar to the first, but one third the size (Wells 1991).

The sum of the squares of any nonadjacent pair of connectors of an orthocentric system equals the square of the diameter of the circumcircle.
Orthocentric systems are used to define orthocentric
coordinates.

The four circumcircles of points in an orthocentric system taken three at a time (illustrated above) have equal radius
(Wells 1991).

The four triangles of an orthocentric system have a common nine-point circle, illustrated above. Furthermore, this circle is tangent to the 16 incircles
and excircles of the four triangles (Wells 1991).