Furnishing element comprising - at least a first and a second module (8,12) interlocking with each other, - wherein the first module is provided with a first wall having a first seat, the first seat having a first main longitudinal extension (S'-S'), wherein the first seat, with respect to a cross-sectional plane perpendicular to the first main longitudinal extension (S'-S'), has a first front branch which emerges on the first wall and at least a first transverse branch incident and contiguous with said first front branch, wherein the first front branch and the first transverse branch define a first undercut in a separation direction F-F of the modules (8,12), - the second module is provided with a second wall having a second seat, the second seat having a second main longitudinal extension (S"-S"), wherein the second seat, with respect to a cross-sectional plane perpendicular to the second main longitudinal extension (S"-S") has a second front branch which emerges on the second wall and at least a second transverse branch incident and contiguous with said second front branch, characterised in that - the furnishing element comprises an attachment key, at least partially counter-shaped to said first and second seats (20,40), - wherein said first and second walls (16,36) face each other so as to align the respective first and second front branches (24,44), - the attachment key having a central section ending in two opposite appendages (58,59), wherein a first appendage has an extension less than or equal to said first branch of the first seat, so as to be inserted through the first front branch in an insertion direction perpendicular to the respective first and second walls (16,36) and locked inside the first seat in a sliding direction (Y-Y) parallel to said first transverse branch so as to misalign at least partially the first appendage with respect to the first front branch and abut the first appendage against the first undercut. - at least a first and a second module (8,12) interlocking with each other, - wherein the first module is provided with a first wall having a first seat, the first seat having a first main longitudinal extension (S'-S'), wherein the first seat, with respect to a cross-sectional plane perpendicular to the first main longitudinal extension (S'-S'), has a first front branch which emerges on the first wall and at least a first transverse branch incident and contiguous with said first front branch, wherein the first front branch and the first transverse branch define a first undercut in a separation direction F-F of the modules (8,12), - the second module is provided with a second wall having a second seat, the second seat having a second main longitudinal extension (S"-S"), wherein the second seat, with respect to a cross-sectional plane perpendicular to the second main longitudinal extension (S"-S") has a second front branch which emerges on the second wall and at least a second transverse branch incident and contiguous with said second front branch, - the furnishing element comprises an attachment key, at least partially counter-shaped to said first and second seats (20,40), - wherein said first and second walls (16,36) face each other so as to align the respective first and second front branches (24,44), - the attachment key having a central section ending in two opposite appendages (58,59), wherein a first appendage has an extension less than or equal to said first branch of the first seat, so as to be inserted through the first front branch in an insertion direction perpendicular to the respective first and second walls (16,36) and locked inside the first seat in a sliding direction (Y-Y) parallel to said first transverse branch so as to misalign at least partially the first appendage with respect to the first front branch and abut the first appendage against the first undercut.