Most people learn their hair type from a number system they discovered online. Type 2c. Type 4a. The numbers are useful shorthand, and the communities that use them have built genuinely valuable practical knowledge around them. But they also suggest more precision than the science of hair curvature actually supports.
Understanding what curl pattern actually is, where it comes from, and how it interacts with other hair properties gives you a more reliable foundation than any system of letters and numbers. It also reveals why two people with the same nominal type can have very different hair in practice.
Where curl comes from
Hair curl is not a surface property. It's determined before the fiber reaches the scalp, in the living follicle.
The hair cortex (the inner structural core of the shaft, which provides strength and makes up the bulk of each fiber) contains two distinct types of cells: paracortical cells, where the protein filaments run roughly parallel to the fiber axis, and orthocortical cells, where those filaments follow a more helical arrangement. The mechanical behavior of these two cell types differs because of their different filament geometry and their different disulfide bond content (disulfide bonds are the cross-links between protein chains that give hair its structural rigidity). When the developing hair shaft dries and contracts as it exits the follicle, these mechanical differences create a stress differential. If the two cell types are arranged symmetrically around the fiber's cross-section, the fiber pulls straight. If they are laterally segregated — one type occupying one side of the shaft more than the other — the fiber bends toward the side with more resistance, producing curl.[3]
The degree of lateral segregation, combined with the fiber's cross-sectional shape, determines how tight the curl is. Straighter fibers tend to have a more circular cross-section. Curlier fibers tend to be more elliptical, with one axis wider than the other, and this ellipticity acts as a synergistic factor in curl formation alongside the cell-type asymmetry.[4] The follicle itself also plays a role: curly hair follicles exhibit retrocurvature (a backward-curving shape in the lower follicle), which adds another physical driver of the coiling pattern.[2]
The genetic architecture behind all this is beginning to be mapped. A genome-wide association study comparing low and high curl individuals identified strong signals at genes encoding trichohyalin (a structural protein important in inner root sheath development), a copper transporter protein called CUTC, and keratin 74, an inner root sheath component.[2] Curl is genetic at its foundation, expressed through these cellular and structural mechanisms.
How curl is measured scientifically
One of the most rigorous approaches to classifying hair curvature was developed by researchers measuring four geometric parameters across hair from 2,449 subjects from 22 countries: curve diameter (how wide the coil is), curl index (a ratio measuring how much the hair deviates from a straight line), and the number of waves and twists per unit of length.[1] Using these objective measurements and statistical clustering, they identified eight distinct groups of hair type that appear across the worldwide population. The classification is deliberately based on shape rather than ethnicity, because hair type does not map cleanly onto racial categories — there is more variation within any broad population group than is captured by dividing the world into three or four groups.
This eight-type system forms the scientific basis for L'Oréal's curl classification, which is one of the four major classification frameworks currently used in clinical and research contexts. A systematic review of hair classification systems published in 2026 identified the André Walker system, the LOIS system, the FIA system, and the L'Oréal system as the four mainstream approaches, each with different strengths and limitations.[5] The André Walker system, which underlies the 1–4 typing commonly used online, uses qualitative descriptors rather than objective measurements, which makes it intuitive but harder to apply consistently across different people.
None of these systems is a universal standard, and none fully captures the diversity of curl patterns found within any single person's head of hair, let alone globally.
The four broad pattern groups
Despite the complexity, the four-category framework (straight, wavy, curly, coily/kinky) does describe meaningfully different fiber behaviors.
Straight hair has a roughly circular cross-section and minimal curvature. Sebum from the scalp travels down the shaft efficiently. The cuticle tends to lie flat, contributing to shine. Straight hair is generally the most resilient to mechanical damage because the fiber experiences less bending stress along its length.
Wavy hair sits in a wide zone between straight and curly. The curvature is present but shallow, with the fiber oscillating in a loose pattern rather than maintaining a tight coil. Sebum distribution is less uniform than on straight hair, and the areas between waves can experience different moisture levels than the wave peaks. Wavy hair tends to be more sensitive to humidity than straight hair, because the partial cuticle lift at each wave increases the surface area exposed to atmospheric moisture.
Curly hair has tighter curvature with a clear coil pattern. The elliptical cross-section is more pronounced, and the fiber's natural bend creates points of stress that make it more prone to tangling, breakage at the curvature points, and difficulty with sebum distribution from scalp to ends. Moisture is harder to maintain along the length than on straighter types, partly because the structure of the fiber slows the wicking of sebum and water outward.
Coily and kinky hair represents the tightest curvatures: small-diameter coils, zigzag patterns, and combinations of both. The highly elliptical cross-section, high degree of orthocortical cell segregation, and retrocurved follicle shape all contribute. Research comparing hair fiber characteristics across populations found that hair of African origin showed the highest lipid content of any type measured, which is relevant to its conditioning and moisture needs, and that cuticle fragility in this hair type is primarily associated with knotting and the crack and flow formations that occur at the tight bends rather than with structural differences in the keratin itself.[4]
“Hair curl isn't a surface property. It's predetermined in the living follicle before the fiber has even emerged from the scalp.”
What no classification captures
A classification system assigns you a type. What it can't do is account for the fact that curl pattern is one dimension among many, and not always the most important one.
The same person can have Type 3b curls at the crown, looser waves at the nape, and tighter coils at the temples. Porosity, density, and strand thickness all vary independently of curl pattern — and in many cases they're more predictive of which products will work than the curl number is. Two people who share a Type 4a classification might have completely different porosity, density, and damage history, and need very different routines.
Curl pattern also changes with water. Dry hair and wet hair can appear to sit in different categories. Many wavy types present as straight when dry; many curly types define into a tighter pattern when soaked. The "true" curl pattern is best assessed on clean, conditioner-free, wet hair with no product applied — which is rarely how people encounter their hair.
Why curl pattern matters for hair care
Curl pattern shapes a few specific practical things.
Detangling: the tighter the curl geometry, the more points of potential friction and tangling between strands. Coilier hair requires more careful detangling, typically with a lot of slip from conditioner and working from ends toward roots rather than root to end.
Sebum distribution: sebum produced at the scalp moves down the shaft by wicking along the surface. On a straight fiber this is easy; on a coiled fiber, each bend is a potential stopping point. This is why dry ends and oily roots often coexist in curlier hair types, and why moisturizing products are needed further along the lengths rather than at the roots.
Product application: coilier patterns need product applied in sections and worked through intentionally, because coverage through a dense curl pattern doesn't distribute passively the way it does on straight hair.
Moisture retention: the cuticle of curved hair tends to lift more readily at the outer bends of each coil, making moisture retention less efficient and porosity more of a variable. This is why porosity matters more in curly and coily hair routines than in straight hair routines, where the cuticle lies naturally flat.
The hair porosity article covers how porosity interacts with moisture retention in more detail, and the hair density and thickness article covers the other key dimensions that curl type doesn't capture.
ROOTS and curl pattern
ROOTS includes curl pattern as part of the hair profile because it shapes how ingredients and formulas interact with the fiber at a structural level. Products that work well for straight hair often don't work the same way on highly coiled hair, even when they share the same ingredient list, because the fiber geometry changes how ingredients deposit, distribute, and perform. If you've taken the ROOTS quiz, your curl pattern is already part of how your product matches are calculated.