1. Consequently, the other four points form an orthocentric system. 2. The incenter and excenters together form an orthocentric system. 3. In an orthocentric tetrahedron the four altitudes are Monge point of the tetrahedron. 4. The center of this common nine-point circle lies at the centroid of the four orthocentric points. 5. Note that the incenter of this common orthic triangle must be one of the original four orthocentric points. 6. Another necessary and sufficient condition for a tetrahedron to be orthocentric is that its three bimedians have equal length. 7. If all three pairs of opposite edges of a tetrahedron are perpendicular, then it is called an orthocentric tetrahedron. 8. The orthocentric point that becomes the incenter of the orthic triangle is that orthocentric point closest to the common nine-point center. 9. The orthocentric point that becomes the incenter of the orthic triangle is that orthocentric point closest to the common nine-point center. 10. When only one pair of opposite edges are perpendicular, it is called a "'semi-orthocentric tetrahedron " '.
