1. Introduction
Collinearities of triangle centers which are defined in apparently different ways has been of interest to geometers since it was first noticed that the orthocenter, centroid, and circumcenter are collinear, lying on Euler’s line. Kimberling [3] lists a great many collinearites, including many more points on Euler’s line. The object of this note is to present a three-dimensional graphical summary of 25 threecenter collinearities involving 17 centers, in which the centers are represented as vertices and edge midpoints of nested polyhedra: a tetrahedron circumscribing an octahedron which then circumscribes a cubo-octahedron. Such a symmetric collection of collinearities may be a useful mnemonic. Probably the reason why this has not been recognized before is that two of the vertices of the tetrahedron represent previously undescribed centers. First we describe two new centers, which Kimberling lists as X1276 and X1277 in his Encyclopedia of Triangle Centers [3]. Then we describe the tetrahedron and work inward to the cubo-octahedron