EK-Classic HDC 12 (Hard Tubing Compression) Fittings are 12mm hard-tube compression fittings designed for use with solid tubing such as Acrylic and PETG. With a high-quality finish, the EK-Classic HDC Fittings are an aesthetically appealing and secure option for your custom loop liquid cooling system. A non-intrusive, minimalistic look, with NO added branding at all.