Curl typing systems that sort hair from straight through coily are now standard vocabulary in American salons. They describe one property well and several others not at all.
What determines curl shape
Curl originates in the follicle. A round follicle growing straight out produces a straight strand, while an oval or elliptical follicle set at an angle produces a curved one.
Inside the strand, the distribution of two keratin cell types is asymmetric in curly hair. Uneven distribution pulls one side tighter and holds the bend in place.
Because the shape is set below the skin, it is a structural property. Styling changes how the curl presents, not what the follicle is producing.
What the numbered system captures
The common scale runs from type one, straight, through waves, curls and coils, with letter subdivisions for how tight the pattern is within each band.
It is essentially a geometry scale, measuring the diameter of the curl loop. That is genuinely useful for choosing a diffuser, a cut or a styling technique.
Most heads carry more than one pattern. The crown, nape and hairline commonly differ, which is why a single number rarely describes a whole person.
Why porosity often predicts more
Porosity describes how readily the cuticle layer admits and releases water. High porosity means a raised or damaged cuticle that absorbs quickly and dries quickly.
That property governs how a conditioner behaves, how long a style holds and how the hair responds to humidity, none of which the curl number addresses.
Chemical processing, heat and mechanical wear all raise porosity, so it changes over a lifetime while curl pattern largely does not.
How strand thickness enters the picture
Individual strand diameter is independent of curl shape. Fine coily hair and coarse coily hair need different product weights despite sharing a classification.
Density, the number of strands per area of scalp, is a third independent variable. Fine strands at high density can look thick while behaving as fine hair.
Choosing a product on curl number alone therefore ignores two of the three variables that determine whether a cream will weigh the hair down.
Where the system falls short
The scale was popularized commercially rather than developed as a clinical measure, and its subdivisions are judged by eye without a defined standard.
It also encourages sorting textures into a hierarchy, which has a fraught history in the United States and is worth separating from the practical question of styling.
Sudden pattern change, patchy loss or scalp pain are medical questions. A dermatologist, not a typing chart, is the right next step for those.