This series fittings allow for the connection of cross-linked polyethylene pipe (PE-X). On the coupling ends, the union with the polyethylene tube is sealed by O-rings placed on the removable insert that is introduced into the fitting body and into the tube. The assembly is tightened by the nut that compresses the clamp, leaving the tube subject and the connection tight. In the different articles of the family the references with different sizes are identified in inches for the threaded end, and in millimetres for the coupling end in reference to the outer diameter and thickness of the corresponding polyethylene tube. Examples of applications may be plumbing for water supply and distribution, hot water feeding for radiators and underfloor heating.