There are several classifications for hair types and textures, but one commonly used system categorizes them into the following categories: Straight hair: hair that is smooth and lies flat, with no wave or curl. Wavy hair: hair that has a slight wave pattern, but is not curly or straight. Curly hair: hair that has a definite curl pattern, ranging from loose spirals to tight ringlets. Coily hair: hair that forms tight, compact spirals or coils. In addition to these main categories, there are also subcategories that take into account the thickness, porosity, and density of the hair. Some systems include: Fine hair: hair that is thin in diameter and delicate in texture. Medium hair: hair that is neither fine nor coarse, with a moderate diameter. Coarse hair: hair that is thick in diameter and rough in texture. Low porosity hair: hair that resists absorbing moisture and is difficult to penetrate with products. High porosity hair: hair that absorbs moisture easily and is often dry, frizzy, and damaged. High density hair: hair that is tightly packed on the scalp, giving the appearance of fullness. Low density hair: hair that is thinly spaced on the scalp, giving the appearance of thinness.