Abstract: An undermattress which includes a frame which is spanned in a transverse direction by a plurality of laths which are supported on the frame at each end, so as to be yieldable in a direction of loading of the undermattress, by structure that holds a plurality of laths via joints (3,4,7) which are each formed from a joint pin (3) having a spherical or ellipsoidal joint head (4) and a guide bush (7). Each joint head (4) is fitted into a respective guide bush (7) and is shaped for enabling movement of the joint pin (3) relative to the guide bush (3) in every direction. In this way, sufficient freedom of movement is provided to enable the laths to be held without tilting on the frame even under unilateral loading.