Abstract: The invention is concerned with a seal, particularly in a line which is connected to a sanitary appliance such as a shower drain and the sewer It has a passageway with a passage (4) of which a sheet like boundary (2) is easily deformable such that said passageway has a first position wherein the passage is gas tight and a second position wherein the passage allows passage of liquid. Said passage boundary biases said passage in the first position by a force. The passageway (2) is substantially sheath or hose like and the passage (4) is at the outer side thereof and the passage (4) is ring gap like and extends in flow direction (arrow A) over some distance.