The present invention relates to a laminar blank for assembling polyhedra which comprises at least three arrises (2a, 2b). Each of the arrises (2a, 2b) comprises: a first tab (4) projecting from the arris (2a, 2b) and forming a fold line, at least two closure grooves (12) provided in said arris (2a, 2b), a second fold-in tab (6) projecting from the first tab (4) and forming at least one fold-in line (30), a closure flange (10) projecting from the second tab (6) and aligned with a closure groove (12), and a third tab (8) projecting from one of the side edges (22) of the first tab (4) and being foldable with respect to the first tab (4) for cooperating with the adjacent arris (2b). The invention also relates to a polyhedron formed from laminar blanks according to the invention, as well as to a method of assembly.