This screw nut (1) is designed for the rigid fixation of a bone screw (15) to a bone plate (20) having plate holes (21). It has a generally cylindrical body (5,6) with a central axis (2), a central hole (3) with a hole axis (34) and an internal thread (4) designed to receive a bone screw (15). The body (5,6) comprises of a lower section (5), an upper section (6), a lower side (9) and an upper side (10). The profile (7) of the upper section (6) orthogonal to said hole axis (34) is smaller than the profile (8) of the lower section (5) orthogonal to said hole axis (34) and the upper section (6) is shaped to be insertable into the plate hole (21) of the bone plate (20).