Zip fastener with emergency opening facility

In a zip fastener the elements 10 on one tape have a head which engages a shoulder 20 of an opposed element to limit out of plane movement. A small number of elements 30 have a head 38 which bears on a shoulder 46 when twisted in one direction but passes the shoulder 46 when twisted in the opposite direction to allow partial separation of the elements to create a point of weakness. The coupled elements can be peeled apart when sufficient pressure is applied to one side of the tape at the point of weakness, but the elements resist twisting when pressure is applied to the opposite side.