Wig caps, the foundation for wig construction, are sewn onto. These are made of stretchy fabric with elastic around the head. They were sourced from a wig maker in the Philippines. Some are old stock and may have issues like the elastic losing its stretch or having stains. The first photo shows sizes 13-14, 15-16, 16-17, and 17-18 inches from right to left. These are larger sizes that were purchased for testing. The second photo shows an 8-9 inch cap placed on a ball. Regardless of size, they are all constructed with three parts: a center and two side pieces. It's difficult to use a small cap on a larger wig, but the size can shrink during sewing, so it's safer to use a slightly larger cap.