Insert nut for a screw nut joint (26, 11) of the type which comprises a substantially rectangular block, of which the short side substantially corresponds to the width of the outer groove portion (15) in a T-formed groove (13), which outer groove portion (15) is delimited by side flanges (16) in a profile (12) or the like and which inner groove portion (17) is expanded relative to the outer groove portion (15). The long side of the insert nut (11) substantially corresponds to the width of the inner groove portion (17), and which nut by rotation can be transversally positioned and be brought into contact with the underside of the side flanges (16) of the groove. The nut (11) is designed with a substantially quadratic shoulder section (20), which is arranged concentric with the threaded hole (19) of the nut, and with substantially the same width as the short side of the nut. The shoulder section is intended to cooperate with the side edges of the outer groove portion (15), and it is divided in the vertical direction in a lower quadratic portion (21) and an upper quadratic portion (22) designed with two diametrically opposed rounded corners (23).