Abstract: A seal for reusably sealing an opening formed by a slit in a sheet material portion of an envelope providing an airtight enclosure, which seal comprises an edge formation having first and second flexible members secured along respective first and second edge portions of the slit, a rigid member extending along the length of the slit and having a channel along its length for receiving and holding therein the first and second flexible members in contact with one another so as to close the opening, inflatable means operable upon inflation thereof to urge the first and second flexible members into sealing engagement with one another and to secure the first and second members within the channel of the rigid member, and retaining means for resisting withdrawal of the first and second flexible members from the channel whilst the inflatable means is deflated. The retaining means simplifies the alignment of the flexible members prior to inflation.