Abstract: A pipe coupling, which renders it possible to connect branch pipes (24) to a feed-pipe (1) without requiring the flow in the feed-pipe to be shut off. In a cylindrical guide passageway (5) of one of two coupling parts (2a,2b) embracing the feed-pipe a piston (4) is located, which is provided with a passageway (10) for fluid connection between the feed-pipe and the branch pipe. The piston is provided with an internal knife (9) which upon rotation of the piston cuts up material out of the feed pipe to form the opening to the passageway and projects the cut-up material into the guide passageway (5) to assist in anchoring the coupling in place.