1. N. Altshiller-Court: Notes on the orthocentric tetrahedron. Amer. Math. Monthly 41 (1934), 499–502.
2. N. Altshiller-Court: The tetrahedron and its altitudes. Scripta Math. 14 (1948), 85–97.
3. N. Altshiller-Court: Modern Pure Solid Geometry. Chelsea Publishing Co., New York, 1964.
4. M. Berger: Geometry I. Springer, Berlin, 1994.
5. G. Berkhan, W. F. Meyer: Neuere Dreiecksgeometrie. In: Encykl. Math. Wiss. III, AB 10, Leipzig, 1913, 1173-1276.