Which type of perm rod has a small diameter in the center and gradually increases in diameter along its length?

The type of perm rod characterized by a small diameter in the center that gradually increases in diameter along its length is known as the concave rod. This design allows for a tighter curl at the center of the hair shaft, while the increased diameter towards the ends creates a softer, more blended curl. This unique shape helps to achieve a variety of textured styles, giving the stylist versatility based on the desired outcome.

In contrast, straight rods maintain a uniform diameter throughout their length, which produces consistent curls without the gradient effect seen in concave rods. Cylindrical rods have an even, cylindrical shape and result in standard curls, and spiral rods are typically designed to create corkscrew-like curls due to their longer, spiral design. Each of these other types serves specific styling needs, but they do not incorporate the gradual increase in diameter that defines the concave rod.