Abstract: A coupling has a fastening section held in a hole of a first board-shaped material, a fixing section held in a holding hole of a second board-shaped material, and two detaining sides on two ends of the fastening section, which are tightly pressed against respective ones of two sides of the first board-shaped material; the fixing section has several flat ringed parts encompassing and spaced along it, which are slightly wider than the holding hole, therefore the fixing section will be tightly held in the holding hole; the first detaining side includes a belt-shaped plate part, which encompasses the fastening section, and has first and second ends, between which a gap exists; in assembling, the coupling is held in such a position as not to be perpendicular to the first board-shaped material for allowing the first detaining side to pass through the hole of the first board-shaped material.