Size of Book Ring: The book binding ring has an outer diameter of 50mm, an inner diameter of 44mm, and a thickness of 3mm. It is small in size and easy to carry. Safe Material: The black curtain ring is made of high-quality metal, anti-rust, durable, smooth and close tightly, suitable for storing loose items. Easy to use: The rings of the binder are connected by rotating rivets. When opening, just gently lift and pass the required books through the final press to close the ring. Multipurpose: Connect and organize loose files, books, materials into a sturdy keychain, so that your items will never be lost again. Package Included: You will receive 20 pcs metal book rings 50 cm 2 inches, super large size, meet daily needs.