Permanent wave rods that produce a tighter curl in the center and looser curl on either side of the strand are

• A. Concave rods
• B. Convex rods
• C. Spiral rods
• D. Straight rods

Answer: (A)

Concave rods create this specific curl distribution because their middle is narrower than the ends. Wrapping hair around a smaller diameter forces it to bend more tightly in the center, giving a tighter curl there, while the ends wrap around a larger diameter, producing looser curls. If you used convex rods, the center would wrap around a larger diameter and the ends around a smaller one, reversing the effect. Straight rods would give uniform curl along the strand, and spiral rods produce a twisting texture along the length rather than a centralized tight curl.