A spring mattress is disclosed including a plurality of strings interconnected side by side, each string including a plurality of continuous casings and each casing including a coil spring. Further, in at least one embodiment, at least one of the casings in at least one of the strings includes a plurality of stretch openings into the interior of the casing, wherein at least some of said stretch openings are displaced in relation to each other in a lengthwise direction of the spring. At least one embodiment of the present invention also relates to a method and a device for manufacturing of such a mattress, as well as to a bed adapted for such a mattress.