The Nagel line is the term proposed for the first time in this work for the line on which the incenter I, triangle centroid G, Spieker center Sp, and Nagel point Na lie. Because Kimberling centers X_1 and X_2 both lie on this line, it is denoted L(X_1, X_2) and is the first line in Kimberling's enumeration of central lines containing at least three collinear centers.

incenter | Nagel point | Spieker center | triangle centroid