The coupler (100) comprises a pair of U-shaped support members (112) having pivoted thereto an inverted U-shaped saddle (120). The support members (112) and saddle (120) intersect at (128) in a clamping condition to clamp a tube (124) in the coupler and outwardly extending arms (130) and (132) of the support members and saddle respectively may be urged apart by a wedging device (134) to provide the clamping force. The wedging device (134) is a toggle joint which in the undercentre condition (160) illustrated wedging engages the arm (132) which projects through an opening (146) in a locking arm (144). The arm (132) is radiused about an outwardly offset axis (152) relative to the pivot axis (137) of toggle joint (134). The toggle joint may be moved to an overcentre condition (162) in which the opposed edges of the opening (146) lockingly engage the upper and lower surface (150) and (151) of the arm (132), which may be serrated as at (148). The coupler (100) may be interlocked with a second identical coupler (100').