3/8 line x 3/8 line Bulkhead union fitting. This fitting will work with any type of line. Plastic, Nylon, Copper, and Stainless. Will work with fluid, compressed air, nitrogen, helium, any inert and non-combustible gases. For a simple and quick assembly of pneumatic circuits made from nickel plated brass. O-ring is a silicone-free nitrile.