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There's no doubt curls are the way to go style-wise, but loose curls suit some fashionistas better than tight spirals. To get a loose look with your tresses when you get a perm, you have to pick your perm rods carefully. Loose curls are possible with all but the smallest rod size, but your hair length plays a big role in whether the curls end up tight or relaxed.
The Universal Standard
Perm rod sizes are based on a universal standard. The different sizes are color-based and include red, blue, pink, gray, white and purple rods. The red rods are the smallest and are used only to make tight curls; only gals with hair 2 1/2 inches or less can rock these rods. Tight curls are made with blue rods for hair 2 to 3 1/2 inches long, pink rods for hair 3 to 4 inches, gray rods for hair 3 to 5 inches and white rods for hair 4 1/2 to 6 1/2 inches. Purple rods are the biggest size and generally only produce waves, so they're sometimes called "body rods." The general rule of thumb is that if you want a looser curl, you use rods at least one size bigger than the ideal for your hair length. For instance, if your hair was in the 2- to 3 1/2-inch range and thus ideally required blue rods, you'd use the rods ideal for the 3- to 4-inch range, or the pink rods. The further away you go from the standard, the looser your curls will be.
Why It Works
The principal behind using a slightly larger rod than your ideal size to get a looser curl is simple: the bigger the rod is, the larger diameter and circumference it has. Thus, the fewer times your tresses can wrap around it.
Even though using one size rod larger than your normal one is a good way to produce a loose curl, your hair type also plays a role in how loose the perm ends up. Some hair is naturally "hard to perm." For ladies with this problem, even if you choose the ideal rod length, the perm might not take as well and the curl could end up looser than it would for others.
Whenever you're in doubt about what rods to use, always check with your stylist. You just need to be clear about the degree of looseness you want. Bring in a picture of the style you covet so that there's no confusion. Your stylist can use other techniques such as wrapping very loosely if you want a looseness that's somewhat between rod sizes, too. She even can use nonstandard tools to wrap your hair to get the proper diameter.