The machined steel Martin link is the correct thickness to fill the entire slot when used with a black box or blue box. While some martin links are thinner and rattle around, this one will fit perfectly. When used without a black or blue box, just wrap around subject's waist, insert Martin link through any link of chain at the appropriate tightness, and thread handcuffs through link to form a secure and easy-to-use waist restraint. 72" long with snap clip at end to tie off excess chain. The chain and link are completely nickel plated for durability and rust resistance.