Measurement · LGON
Lower gonial angle (N-Go-Gn)
Loading the reference ceph.
Definition
The lower portion of the gonial angle: the angle at Gonion between the line to Nasion and the mandibular corpus (toward Gnathion).
How
At Gonion (Go), measure between Go→N and Go→Gn (along the lower border of the mandibular body). Upper + lower gonial angles equal the full gonial angle (Ar-Go-Gn).
Purpose
The lower gonial angle is Jarabak's strongest single growth-direction predictor: it reflects the inclination of the mandibular body itself. High values point to vertical (clockwise) growth, low values to horizontal (counterclockwise) growth.
High
Above the norm reads as a steep mandibular body — vertical (clockwise) grower; expect the chin to swing down and back.
Low
Below the norm reads as a flat mandibular body — horizontal (counterclockwise) grower; the chin comes forward with growth.
This is a reviewed tracing on one real cephalogram — the same ceph on every page in Learn — so Lower gonial angle (N-Go-Gn) here is a real reading and not a diagram.