A buckle comprising a female part (12; 212) and a male part (14; 214), each having means (22; 24', 24"; 222; 224', 224") for connection to corresponding elements of articles of clothing or accessories to be fastened or coupled together, wherein said female part (12; 212) is provided with a cavity for the insertion of an axial tab formation (30; 230) which is carried by the male part (14; 214) and which has transverse seats (36a, 36b; 236a, 236b) for engagement by fastening means associated with the said female part (12; 212) and projecting into the cavity, these means being adapted to assume a locking position in which they engage in the said seats (36a, 36b; 236a, 236b) so as to prevent the sliding of the male part (14; 214) and the female part (12; 212) relative to each other, in a fastened condition of the buckle, and a release position in which they are disengaged from the said seats (36a, 36b; 236a, 236b), thus permitting the relative sliding of the tab formation (30; 230) in the cavity, in an unfastened condition of the parts of the buckle, in which: - the said tab formation (30; 230) has at least one pair of the said seats (36a, 36b; 236a, 236b) for receiving the said fastening means, formed at the opposite lateral edges (34a, 34b; 234a, 234b), - the said fastening means comprise a pair of teeth, located on opposite sides of a longitudinal direction of coupling of the buckle, which define between them a transverse aperture for the insertion of the tab formation (30; 230), and - the buckle comprises button-like release means (42; 242) connected to the said fastening means and operable manually by pressing in a direction transverse to the buckle to disengage the said fastening means from the seats (36a, 36b; 236a, 236b) of the tab formation (30; 230), the fastening teeth and the release means (42; 242) acting in coinciding or parallel transverse directions, so that the simultaneous pressing of the buttons (42; 242) towards each other causes the fastening teeth to move away from each other, thus expanding the said transverse aperture, in other words causing the fastening teeth to move away from the engagement seats (36a, 36b; 236a, 236b) on the tab formation (30; 230). characterized in that the buckle comprises a separate operating mechanism (40; 420) carrying the said fastening means, which can be coupled removably to the female part (12; 212), this mechanism including a pair of movable fastening and operating members (70a, 70b), positioned at least partially facing each other and offset in the transverse direction, each of these carrying a corresponding fastening tooth and forming the corresponding operating button (42; 242), the buttons being located on opposite sides of the longitudinal direction of the buckle, and the said pair of fastening and operating members (70a, 70b) being joined by resiliently deformable joining members in such a way as to permit their transverse relative movement to move the fastening teeth towards or away from each other, so that the said mechanism (40; 240) forms an integral structure with a variable transverse extension. - the said tab formation (30; 230) has at least one pair of the said seats (36a, 36b; 236a, 236b) for receiving the said fastening means, formed at the opposite lateral edges (34a, 34b; 234a, 234b), - the said fastening means comprise a pair of teeth, located on opposite sides of a longitudinal direction of coupling of the buckle, which define between them a transverse aperture for the insertion of the tab formation (30; 230), and - the buckle comprises button-like release means (42; 242) connected to the said fastening means and operable manually by pressing in a direction transverse to the buckle to disengage the said fastening means from the seats (36a, 36b; 236a, 236b) of the tab formation (30; 230),