Straight wavy and curly hair are all normal examples of normal hair presentation. Healthy hair will have an outer cuticles made from dead, keratinized cuticle cells that over lap each other in a roof tile effect. The outer cuticle is the strongest part of the hair fiber and protects the inner cortex. The difference between straight, wavy, and curly hair is in the shape.
Straight hair is circular, or nearly circular in cross section.
Wavy hair has a more oval cross section and curly hair has a very flat oval cross section. The circular cross section of straight hair means it has optimum strength in any direction transversely across the fiber. Because wavy and curly hair fibers are oval in cross section, they are stronger and less flexible in one direction but more flexible across the flatter side of the oval cross section. The result is curls in hairs adjacent to each other are all aligned together so the curls all run the same way. Because the hair follicles in the skin are all aligned in the same direction.