Chapter 1: The Hair Type Classification System
The Andre Walker Hair Typing System classifies hair into four categories: Type 1 (straight), Type 2 (wavy), Type 3 (curly), and Type 4 (coily). Each has subcategories (A, B, C) indicating the tightness of the pattern. While this system has become the dominant framework, it's incomplete: it describes curl pattern but says nothing about porosity, density, or strand thickness — three variables that often matter more for product selection than curl pattern alone.
Chapter 2: Fine, Medium & Coarse — Strand Diameter
Strand thickness determines how much protein and moisture a single strand can absorb and retain. Fine hair (less than 60 microns) has fewer cuticle layers, less structural integrity, and is more prone to damage. It responds poorly to heavy products and is easily weighed down. Coarse hair (greater than 90 microns) has more cuticle layers, is more resistant to chemical services, and can handle heavier, richer products.
The thread test: pull a single strand from your head and hold it alongside a piece of thread. Thinner than the thread = fine hair. Similar thickness = medium. Thicker than the thread = coarse.
Chapter 3: Porosity — High, Low, and Normal
Porosity describes your hair's ability to absorb and retain moisture — determined by the structure of the cuticle layer.
Low porosity: Cuticle layers lie tightly closed. Water beads on the surface. Products sit on top rather than absorbing. Hard to color. Needs heat to open the cuticle for deep conditioning treatments to penetrate.
Normal porosity: Cuticle opens and closes appropriately. Absorbs and retains moisture well. Colors evenly. Responds predictably to most products.
High porosity: Cuticle is raised or damaged — gaps allow rapid moisture absorption but equally rapid loss. Hair dries quickly but doesn't stay moisturized. Frizzy, dull, prone to tangling. Usually the result of chemical damage, heat damage, or natural genetics.
Chapters 4–7: Density, Elasticity & Building Your Routine
Density is the number of individual strands per square inch of scalp. High-density hair benefits from lighter products to prevent heaviness; low-density hair benefits from volumizing formulas.
Elasticity is tested by stretching a wet strand between two fingers. Healthy hair stretches to 30% of its length and springs back without breaking. Poor elasticity — where hair stretches and snaps — indicates protein deficiency or significant bond damage. These strands need bond-building treatment before additional moisture.
The most common product mistakes by type: Fine hair loaded with heavy oils (weights strands down). High-porosity hair washed with harsh sulfate shampoos (strips remaining moisture from already-compromised cuticle). Low-porosity hair treated with protein-heavy products without heat to open the cuticle (protein just sits on the surface). Coily hair types washed daily with non-moisturizing formulas.