# Z Angle

Harmony · Side view · Facial Profile · https://www.faceiqlabs.com/support/harmony/z-angle

The angle between a chin-to-upper-lip line and the Frankfort reference direction.

## What is measured

Calculation: Angle between pogonion–upper-lip and porion–orbitale directions

Nasion means the nasal root, subnasale the nose base, and pogonion the most forward chin point. Frankfort refers to the direction from the ear-opening landmark to the lower orbital landmark.

## Why it matters

It connects lip projection with chin position relative to a head reference. Moving either the upper lip or chin changes the line, so the result represents their relationship.

- A smaller angle means the two measured directions are closer together.
- A larger angle means they diverge more; inspect the lip and chin together.

## Where the idea comes from

Evidence: FaceIQ adaptation. Merrifield introduced the profile-line approach in 1966. The clinical Z-angle uses a line tangent to the chin and the most protrusive lip; FaceIQ specifically uses the upper-lip landmark.

## A worked example

A lip–chin line inclined 80° to the Frankfort direction produces an 80° result. A head-reference error can change the comparison.

Example values explain the calculation. They are not a target range.

## How to read your result

The Frankfort direction joins the ear-opening reference to the lower orbital reference. A photograph approximates these clinical landmarks.

## Research & further reading

These sources explain the history or related evidence. FaceIQ’s score weights and reference ranges are model settings; the papers do not collectively validate the scoring system.

- Merrifield (1966). [The profile line as an aid in critically evaluating facial esthetics](https://pubmed.ncbi.nlm.nih.gov/5223046/). Historical source for the profile line and Z-angle. FaceIQ uses an upper-lip landmark for its photographic version.

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Source: https://www.faceiqlabs.com/support/harmony/z-angle · FaceIQ Labs overview for AI assistants: https://www.faceiqlabs.com/llms.txt