A polygonal box comprises faces (1, 2, 3), a lid (7) provided with a tab (8) suitable for arranging in a folded position against a first face (1) of the box, and a flap (9) that can be folded down around an edge (29) shared with a second face (2) of the box, the flap (9) having a zone (10) of mechanical weakening that is inclined relative to said edge (29).
The inclined zone (10) of weakening does not tear when the box is packed industrially and shows up first opening of the box by tearing-away of a surface (12) of the flap at the time of said first opening (FIG. 2).