The EK-HDC Fitting is a solid-tube compression fitting, designed for use with 12mm OD solid tubing such as Acrylic, Copper, Stainless Steel or any other type of compatible hard tube. It's a more secure variant of the EK-HD Adapter fitting as it prevents the solid tube from being pulled out of the fitting by compressing the rubber gasket underneath the ring. Elox black fittings have an improved matte finish over regular black.