Each Orthocenter of HAB, HAC and HBC is also a vertex of the triangle
ABC.
Note that each Circumcircle intersects another object only at a vertex
of the triangle ABC or at the Orthocenter of that triangle.
The nine point circle for all triangles is the same circle (in green).
The Centroid is collinear with the Orthocenter and the Circumcenter. This
line of collinearity is named the Euler line, after the renowned Swiss mathematician
Leonard Euler who studied the properties of the nine point circle.