The screw has a recess (2) for engaging a tool in its head (1a). The recess encompasses a central recess (4) which is located in the middle of the head and four engaging grooves (3) in connection with the central recess and extending crosswise about the central recess. An abutting top surface (5) forms a part of each engaging groove and extends in the direction of the axis of the screw, whereby the abutting surface is built to be pressed against by the end of a tool when the toll is turned in a direction tightening the screw. A first release top surface (6) is built between each neighbouring pair of grooves and can come into engagement with a front edge of the end of the tool when the tool is turned in an opposing direction. A second release surface borders the first.
