The cramp is designed to join two modular form panels (4) with sections having side recesses (4') on which the cramp actually works, the latter comprising three parts, the first part defined by a tubular section (1) with which two side jaws (2) are integral, another part comprising a travelling section (7') on the mounting section (1), and having another pair of jaws (7) facing the jaws (2) and both having facing heels (3) that are housed in the recesses (4') of the sections (4) to join, the third part comprising a wedge (8) that crosses windows provided in both the sides of the mounting section (1) and the wings of the travelling section (7'). The wedge (8) is T-shaped and one of its wings (11) has a slanting edge that acts as the wedge itself, whereas the other wing defines ribs (12) on either side of the wing (11) to be housed in notches (6) with which the windows on either side of the mounting section (1) are provided to such end. The said notches (6) are slanting and offset on one side with regard to the other, in order that the wedge (8) can be inserted straight in its transverse disposition, the said wedge (8) having a tailpiece (13) at its free end to prevent the same from becoming detached.