The Gergonne line is Oldknow's term for the perspectrix of a contact triangle Δ D E F and its reference triangle Δ A B C. It is determined by the Nobbs points D', E', and F'. In addition to the Nobbs points, the Fletcher point (intersection with the Soddy line) and Evans point (intersection with the Euler line) also lie on the Gergonne line. Furthermore, the Soddy line and Gergonne line are perpendicular.

contact triangle | Euler-Gergonne-Soddy triangle | Euler line | Evans point | Fletcher point | Nobbs points | Soddy line | tangential triangle