Hair textures:

Coarse - fat individual hairs. Can be wiry and strong, or porous (coarse and porous is common in East Asia)

Fine - thin individual hairs.

Porosity depends on the outermost layer of hair, the cuticle, which you can imagine like scales on a snake:

Low porosity is resistant hair, with a tight cuticle, no gaps between the scales.

Medium porosity is hair with a moderately tight cuticle. Often tight healthy curls are medium porosity because the curves of the curls can make these gaps.

High porosity hair has many gaps in the cuticle. Bleached or relaxed hair is high porosity because the chemicals lift the cuticle to do their work and some scales are lost or permanently lifted. But hair can also be naturally high porosity.

Density is the number of hairs per square cm on your scalp. High density is more hair per area.

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