Measurement · ABPLANE
A-B plane angle
Loading the reference ceph.
Definition
The angle between the A–B line (the denture-base line) and the facial plane (N–Pog) — Downs' read of how the apical bases relate to the facial profile line.
How
Draw the line through A and B points, and the facial plane N–Pog; measure the crossing angle between them.
Purpose
Relates the two denture bases to the chin line: it anticipates how easy a canine/molar correction will be — a large discrepancy means the apical bases themselves diverge from the profile and camouflage will show.
High
Above the norm reads as greater A-B discrepancy against the facial line (Class II-type base relation).
Low
Below the norm reads as the bases align with (or reverse against) the facial line (straight or Class III-type).
This is a reviewed tracing on one real cephalogram — the same ceph on every page in Learn — so A-B plane angle here is a real reading and not a diagram.