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Rod size directly determines the curl diameter achieved after reduction and neutralization. Selecting a rod that matches the desired finished curl requires accounting for hair elasticity (ability to stretch and return), texture, and anticipated relaxation during neutralization and after first shampoos. Finer textures may wrap more tightly, while coarse or resistant hair might need smaller rods or alkaline formulations to achieve similar results. Hair color per se does not dictate rod size, though prior lightening affects elasticity and thus tension and formulation. Water temperature is important for thorough rinsing but doesn’t set curl size. Proper wrapping tension, even sectionings (no wider than rod), and end-paper control prevent fishhooks and ensure uniform processing. Test curls during processing verify bond reduction; neutralizer timing locks in the pattern. Education standards stress the interplay of chemistry, tool selection, and hair structure to produce predictable, healthy results with minimal damage.