The UPVC Coupler allows pipes of different sizes to be linked together. These fittings are commonly used to connect small consumer or process feeder lines to big supply circuits or install small diameter connections. Coupler is a small device for connecting two pipes, sometimes called an inline coupler. UPVC Coupler is a very short length of pipe or tube, with either socket or female threads at one or both ends that allow two pipes or tubes of equal or different sizes to be joined together.