Nine Point Circle Theorem

Theorem
Let $\triangle ABC$ be a triangle.

These $9$ points:
 * the feet of the altitudes of $\triangle ABC$
 * the midpoints of the sides of $\triangle ABC$
 * the midpoints of the lines from the vertices of $\triangle ABC$ to the orthocenter of $\triangle ABC$

all lie on the circumference of a circle:


 * 9PointCircle.png

Also known as
This theorem is also known as the nine point circle theorem or the nine-point circle theorem -- there is little consistency in the literature.