The thorns themself have an inner diameter of about 24-25mm. Then It can through the thorn-ring and lock the two rings together. Anti-Slip Ring available with three size:33mm,35mm and 40mm The inside diameter of the bottom ring with lock: about 40mm / 45mm / 50mm (for your choice) Material: High Quality Medical Grade Stainless Steel 316 L, no chrome plating, no rust guaranteed. To ensure proper fit, measure the width of your cock (take that measure at the base). Once you know your diameter, you can choose the size that will fit you the most, by its size and comfort.