Nut for accommodating the head of a fastening element or for inserting into the same

A socket and/or insert (6) for accommodating a fastener head is inserted in a screwing attachment (1). The socket and/or insert (6) comprises two jaws (4, 5) which engage in a tubular holder (3) in which they are locked against rotation. The jaws can slide in axial direction relative to the holder (3). In their one end position, the jaws (4, 5) can be forced apart in radial direction for insertion of a fastener head. Under the effect of axial force in the direction of the arrow (23), the jaws (4, 5) are pressed into the holder (3), whereby the jaws are necessarily squeezed toward each other in radial direction by virtue of the tapered camming faces (12, 13), so that the inserted fastener head becomes firmly clamped.