into and out of the mattress however many alternative locations of the perforations 23 are conceivable for example such an alternative is illustrated in fig2 in this embodiment holes or perforations 23 are likewise arranged so as to open into the gaps that arise between neighbouring pocket units but are also arranged in the upper side and or the underside of the spring units that is on or near the ends of the spring units in the specific embodiment shown the perforations are arranged in the upper side of the mattress this embodiment is suitable for instance when the spring units are configured so that the spring turns in use can block the perforations when positioned just in front of these in the embodiment in fig2 this problem is effectively eliminated since here the perforations are arranged on casing portions which are arranged between spring turns of different sizes moreover the perforations are in the shown embodiment not arranged symmetrically scattered but are offset toward the centre line of the string preferably toward a longitudinal connecting line which is arranged there this is advantageous since for natural reasons some excess casing material is collected near this connecting line which further reduces the risk of the perforations being blocked in use it is also possible to use different pockets with different resistances to air flowing into and out of the pockets in different zones of the mattress an example of a mattress with such different zones is illustrated in fig3 in this embodiment the pocket units in a zone 71 along the edge of the bed are configured to have a higher resistance to air flowing out of the pockets than the other pockets of the mattress this reduces the.