A tightening square with fixed pins (12, 22) for connection of two adjacent sections disposed at right angle; each section (M1, M2, T1, T2) comprising a first internal wall wherein a hole (F1, F2) is obtained for penetration of one of said fixed pins (12, 22), and a second internal wall opposite to said first internal wall; comprising: a) a first plate and a second plate connected in such manner to define a vertex of the square and adapted to be inserted into two end sections of a pair of adjacent sections (M1, M2, T1, T2); each plate having an external side (1 a, 2a), an internal side (1 b, 2b), a first free end (11, 21) and a second end (13, 23) in correspondence of the vertex of the square; b) two fixed pins (12, 22) disposed in correspondence of the first free end (11, 21) of each of said two plates (1, 2); c) adjustment means (14, 24, 4) adapted to get said two plates (1, 2) mutually closer or farther, adjustment means (14, 24, 4) comprising: c1) a first wing obtained on said second end of the first plate and a second wing obtained on said second end of the second plate; c2) a hole obtained in said first wing, c3) a non-threaded hole obtained in said second wing and aligned with respect to said hole of the first wing; and c4) an adjustment screw provided with axis (X-X) and inserted into said non-threaded hole of the second wing and into said hole of the first wing; wherein a clearance is provided between said non-threaded hole and said adjustment screw, which allows the axis (X-X) of the adjustment screw to oscillate along a plane perpendicular to both internal sides (1 b, 2b) of said two plates (1, 2) in such manner that the square can assume a diverged position, in which said two internal sides (1 b, 2b) of said two plates (1, 2) are tilted by an angle (α) higher than 90°; characterized in that said hole of the first wing is a threaded hole and said square comprises an elastic element that comprises a helical spring wherein said adjustment screw is inserted, said helical spring being positioned between said two wings (14, 24) in such manner to oppose the diverging movement of the square and dispose the two plates (1, 2) of the square in mutual orthogonal position. a) a first plate and a second plate connected in such manner to define a vertex of the square and adapted to be inserted into two end sections of a pair of adjacent sections (M1, M2, T1, T2); each plate having an external side (1 a, 2a), an internal side (1 b, 2b), a first free end (11, 21) and a second end (13, 23) in correspondence of the vertex of the square; b) two fixed pins (12, 22) disposed in correspondence of the first free end (11, 21) of each of said two plates (1, 2); c) adjustment means (14, 24, 4) adapted to get said two plates (1, 2) mutually closer or farther, adjustment means (14, 24, 4) comprising: c1) a first wing obtained on said second end of the first plate and a second wing obtained on said second end of the second plate; c2) a hole obtained in said first wing, c3) a non-threaded hole obtained in said second wing and aligned with respect to said hole of the first wing; and c4) an adjustment screw provided with axis (X-X) and inserted into said non-threaded hole of the second wing and into said hole of the first wing; wherein a clearance is provided between said non-threaded hole and said adjustment screw, which allows the axis (X-X) of the adjustment screw to oscillate along a plane perpendicular to both internal sides (1 b, 2b) of said two plates (1, 2) in such manner that the square can assume a diverged position, in which said two internal sides (1 b, 2b) of said two plates (1, 2) are tilted by an angle (α) higher than 90°; characterized in that said hole of the first wing is a threaded hole and said square comprises an elastic element that comprises a helical spring wherein said adjustment screw is inserted, said helical spring being positioned between said two wings (14, 24) in such manner to oppose the diverging movement of the square and dispose the two plates (1, 2) of the square in mutual orthogonal position. c1) a first wing obtained on said second end of the first plate and a second wing obtained on said second end of the second plate; c2) a hole obtained in said first wing, c3) a non-threaded hole obtained in said second wing and aligned with respect to said hole of the first wing; and c4) an adjustment screw provided with axis (X-X) and inserted into said non-threaded hole of the second wing and into said hole of the first wing; wherein a clearance is provided between said non-threaded hole and said adjustment screw, which allows the axis (X-X) of the adjustment screw to oscillate along a plane perpendicular to both internal sides (1 b, 2b) of said two plates (1, 2) in such manner that the square can assume a diverged position, in which said two internal sides (1 b, 2b) of said two plates (1, 2) are tilted by an angle (α) higher than 90°;