In the United States, brass knuckles are not significantly regulated at the federal level, but various state, county and city laws prohibit their purchase and/or possession. Some state laws require purchasers to be 18 or older. Most states have statutes regulating the carrying of weapons, and many specifically prohibit brass knuckles or "metal knuckles." Where they are legal, brass knuckles can normally be purchased online or at flea markets, swap meets, and at some sword and weapon shops. Some companies manufacture belt buckles or novelty paper weights that function as brass knuckles and are sold "for entertainment purposes only". Brass knuckles made of hardened plastic, rather than metal, have been marketed as "undetectable by airport metal detectors." A number of states that ban brass knuckles also ban plastic knuckles. New York's criminal law statutes list both "metal knuckles" and "plastic knuckles" as prohibited weapons but provide no definition for either.