On which type of hair is a permanent wave typically most effective?

A permanent wave, commonly known as a perm, is most effective on medium to coarse hair because this hair type has a more robust hair shaft, allowing it to hold the shape of the curl better once the perm chemicals are applied. The structural integrity and strength of medium to coarse hair enable it to endure the chemical process involved in perming without losing elasticity or becoming overly damaged.

Fine hair, while it can take a perm, often struggles to hold the curl as well as medium to coarse hair due to its lighter structure. Curly hair might not need a perm at all, as it is already shaped, and colored hair may be more damaged and thus less ideal for chemical processes like perming. The effectiveness of a permanent wave hinges on the ability of the hair to withstand and respond to the perm solution, making medium to coarse hair the most suitable candidate for achieving lasting results.