A coupling is provided to hold together a cone and socket joint between items of laboratory glassware. A first annular member is positioned about the cone portion of the joint and has a male threaded end. A second annular member is positioned about the socket portion of the joint and has a female threaded end which may be screwed onto the male threaded end of the first annular member. The first annular member has a seat which engages with an elastomeric sealing ring so that as the first and second annular members are tightened together the elastomeric sealing ring is compressed into the external line of junction corresponding to the outermost circumference of contact between the cone portion and the socket portion of the assembled joint. The second annular member locates indirectly behind the external strengthening lip around the mouth of the socket portion by means of an interjacent ring which has a male frustoconical surface. The second annular member has an inwardly directed flange formed with a female frustoconical surface which cooperates with the male surface of the interjacent ring whereby, in the assembled coupling, the axis of the assembled coupling is aligned with the axis of the cone and socket joint and the interjacent ring is urged radially inwards behind the socket portion of the joint.