Elastic

From CosplayWiki

Elastic is used to hold a garment in place while allowing flexible movement of the body. Swimsuit leg openings, waistbands of pajamas, and wrists of full sleeves are a few examples. As the body moves, the garment fabric stretches and gives, yet stays in place. Is is best to use a zigzag stitch when applying elastic to give it more stretch allowance. Also do not forget to hold the elastic stretched to the size it should be at when worn when you are sewing it in place.