Abstract: A coupling for a modular construction system whereby a tubular element is linked to another element, or is supported, characterised by: 1) at least one cup-shaped unit (P, 22, 41, 61, 81, 102) having an open end (16) and a reduced end (12); the open end (16) serving to received an end (E) of a tubular element (T) to enable the end (E) to be pushed to a rest position (18) at the reduced end (12) of the cup-shaped Unit (P), and locking means (30) enabling a tubular element end (E) to be retained against further movement relative to the cup-shaped unit (P); the length (L') of the unit (P) being between one and one half and twice the outside diameter (D) of the tubular element (or of the major transverse dimension of the tubular element if it is of non-circular cross-section); and an aperture (13) at the reduced end (12) of the cup-shaped unit which is no more than one-half the maximum interior diameter (I) or maximum interior dimension of the cup-shaped unit (P); 2) the or each cup-shaped unit (P, 22, 41, 61, 81