Measurement · YAXIS
Y-axis (S-Gn to FH)
Loading the reference ceph.
Definition
The angle between the S–Gn line (sella to the chin point) and the Frankfort horizontal — the direction the chin has grown away from the skull.
How
Draw S→Gn and Frankfort (Po–Or); the Y-axis is the inferior angle between them (norm ≈ 59°).
Purpose
Downs' growth-vector needle: serial increases mean the chin is dropping (clockwise, vertical growth); decreases mean it is coming forward (counterclockwise). A single reading grades vertical vs horizontal facial pattern.
High
Above the norm reads as a vertically directed growth axis — the chin swings down and back (dolichofacial, Class II-vertical tendency).
Low
Below the norm reads as a horizontally directed growth axis — the chin drives forward (brachyfacial / Class III tendency).
This is a reviewed tracing on one real cephalogram — the same ceph on every page in Learn — so Y-axis (S-Gn to FH) here is a real reading and not a diagram.