Achieving different hairstyles is often difficult if your hair is completely straight or if it lacks body. Perms are a way to add waves or curls with the use of perm rods and chemicals. The resulting curl formation depends on a number of factors, primarily the size and the type of rod that the hair is wrapped around. This makes it crucial that you understand the different types of rods and select the one that is right for what you are trying to achieve.

Straight rods are a type of perm rod that has the same thickness from one end to the other to create an even curl pattern. This type of rod is suitable for all hair lengths and comes in sizes that range from small to large. Large straight rods, for example, provide the best results when body waves are wanted. A body wave is a type of perm that gives hair, particularly straight hair, deep waves that add fullness and body to the hair which is useful for a variety of different styles.

Concave perm rods are made so that they are thicker at each end but thinner in the middle. When using this rod to roll a perm, it rests much closer to the scalp than straight perm rods and has a curl pattern that is tighter at the center and looser at the ends. Concave rods are available in both long and short lengths and, according to Milady's Standard Cosmetology, this is the most commonly used type of perm rod.

The loop rod, also referred to as the circle tool or loop tool, is a long, plastic rod that is used to create spiral curls. The length of the rod is approximately 12 inches and after wrapping the hair around it, the ends are snapped together to create a circular shape or loop. After the perming process is complete and neutralization has taken place, the ends are unsnapped and the hair is unwound, leaving a spiral curl pattern in its place. The loop rod is not suitable for shorter length hair and works best with extremely long hair.

Bendable or bender perm rods are a type of foam-covered rod that is easily bent into different shapes. The length is approximately 12 inches and they are made with a wire in the center, which gives it its flexibility. This type of rod is the same thickness from one end to the other and is used to create a number of different curl patterns, including spiral curls. After wrapping the hair the rod is bent or folded by the stylist to prevent the curl from unraveling.