During a perm, what factor controls the size of the curl?

The diameter of the rod is the key factor that controls the size of the curl during a perm. When performing a perm, the size of the curling rod determines how tight or loose the resulting curl will be. A smaller diameter rod will create tighter, more defined curls, while a larger diameter rod will produce looser waves or curls.

This relationship is fundamental in perming techniques, as stylists select the rod size based on the desired outcome for the client's hairstyle. The diameter directly impacts the way the hair processes and shapes around the rod during the chemical treatment, ultimately determining the final curl configuration.

Other factors can influence the overall outcome of a perm, such as hair length, texture, and the type of solution used. However, these elements do not control the size of the curl in the same direct way that the rod's diameter does. Length may affect the way curls fall and how they appear in relation to the hairstyle, texture can influence how well the hair holds the curl, and different solutions can affect the strength and longevity of the perm but do not change the inherent size of the curl created by the rod.