The modern permanent wave, or perm, is a method of hair treatment that has fallen in and out of favor since it was created in 1938. The perm process uses chemicals to break hair bonds, which can result in luxurious curly hairstyles. A few basic types of curly perms are used at today's salons.

The traditional perm uses chemicals such as ammonium thioglycolate or sodium thioglycolate. Ammonium thioglycolate, or perm salt, is produced from thioglycolic acid -- a weak acid -- and ammonium -- a weak base. A stylist first wraps a client's hair around perm rods, then applies a cream containing the chemical. The lotion breaks down the keratin in the hair, causing it to curl around the rods. An oxidizing lotion is applied to strengthen and set the hair. This method produces strong, full curls.

acid perms use a mild acid -- glycerol monothioglycolate -- instead of perm salt to produce a looser, more flexible curl. These perms are ideal for people who have hair that is sensitive to traditional perm chemicals or who have finely textured hair. Unlike traditional perms, acid perms require heat for the process to work, so the wearer must sit under a hair dryer while the perm is setting.

Exothermic perms use chemicals that create heat as they cause the hair to curl. The stylist applies a lotion to the hair that has been set with perm rods, then she applies a cream that activates the chemical reaction. The advantages to these perms are that they allow for faster chemical penetration into the hair, they do not require an external source for heat and they do not need to be timed, which is required for both traditional and acid perms. This method produces very tight, bouncy curls.

The jheri curl is a type of perm hairstyle popular among African-Americans in the United States during the early and mid-1980s. The style is characterized by its closely gathered, loosely curled, shiny appearance. It was invented by hairstylist Jheri Redding as a low-maintenance alternative to permanent relaxer, which was used to straighten curly hair. The process begins with the application of a sodium hydroxide-based rearranging cream that breaks down keratin in curly hair to straighten it somewhat, followed by a second chemical used to strengthen the hair and set the loose curl.