Elastic - Braided
Braided elastic has lengthwise, parallel ridges. Those ridges make this elastic have more "grip"; but it also means that braided elastic tends to narrow as it is stretched. Braided elastic also rolls more easily than woven or knitted elastics, and tends to lose stretch if it is sewn through. For this reason braided elastic is typically recommended for use in casings, not for sewing directly to fabric. But in some casings (like waists) braided elastic isn't the best choice because of its tendency toward rolling. It's better in sleeves, necklines, or other areas where rolling isn't an issue.