The combination of bone structure, body fat percentage, and muscle that determines how defined a jawline appears — and what can and can't be changed through styling.
Bone Structure Is the Foundation
The underlying mandible (jawbone) shape and the angle at which it meets the skull are set by skeletal development and are the primary determinant of a jaw's basic width and angularity — this is the structural layer that all the face-shape categories on this site describe.
Fat Distribution Changes Visibility
Submental (under-chin) and lower facial fat can obscure jawline definition regardless of the underlying bone shape — this is why body fat percentage changes can dramatically alter how defined a jaw appears without any change to the bone itself, and why jawline appearance can fluctuate with weight changes.
Muscle Contributes Modestly
The masseter (jaw) muscle can add some width and definition, and its size varies based on genetics and, to a modest degree, habitual use, though its overall contribution to jaw appearance is smaller than bone structure or fat distribution for most people.
What Styling Can and Can't Do
Haircuts, beards, and frames work entirely at the level of visual perception — adding volume or lines elsewhere to change how the jaw reads in proportion to the rest of the face — rather than changing the jaw itself. That's precisely why shape-matched styling is useful: it's a genuinely effective lever within its actual scope (perceived balance), even though it has no effect on the underlying structure.