PURPOSE: To use all joints laid lengthwise and crosswise in a mesh-like pattern, as bottom joints.

CONSTITUTION: Joints ares composed of joint bodies 8a, 8b, holders 2a, 2b for supporting the joint bodies and a cap 5 firmly fitted in the holders. Further, the holders are formed with cap support plate parts 4a, 4b defining therebetween deep grooves 7 in which a part suspended from the lower surfaces of the main parts of the cap is fitted, in the crosswise center parts of band-like horizontal plate parts 3a, 3b thereof. A part corresponding to the crossing point of the joints which are arranged, crossing together, in a mesh-like pattern, gives a fitting cut-out part in which the second joint 1b ascends up to the heightwise center while the first joint 1a descends down to the same. The joint bodies of the second joint 1b is formed therein with a cut-out having a width equal to the sum of the width of the cap support plate part and the joint bodies on both sides of the latter, the cut-out being extended from the upper end to the lower end of the joint bodies. The horizontal plate part of the holder of the joint are cut out by a length equal to the width of the horizontal part of the other holder, in a part corresponding to the associated on of the crossing points of the mesh-like pattern.