A hermetically sealed flat container for containing loose materials in liquid, pasty, granular or powder form, which is adapted to also assume a stable upright vertical position, while containing said materials, when one edge thereof rests on a base, said flat container comprising: - a first flexible sheet element having a polygonal shape, which lies over a second flexible sheet element having a polygonal shape, said first and second flexible sheet elements being coupled together by welds formed along the perimeter of their polygonal shape, to thereby define a volume for containing said loose materials in liquid, pasty, granular or powder form, - said perimeter defining a plurality of edges (C, C1, C2 C3), one of said plurality of edges forming a bearing edge for the container to rest upon said base; - said bearing edge progressively reduces its transverse dimensions toward one end thereof with a taper angle (α) with respect to a vertical axis (X-X), such that, when the bearing edge rests on the base, one component of the weight force of the material contained in the container applied to said bearing edge causes a reaction that has the same intensity as said component directed toward the interior of the container, thereby causing said bearing edge to retract into said volume and imparting vertical stability to the container, - wherein said bearing edge comprises at least two mutually transverse sides (T1, T2, T3), which are inclined with a respective one of said taper angle (α) with respect to said vertical axis (X-X) of said container, such that, when said bearing edge rests on said base, the component of the weight force of the material contained in the container, applied to said at least two mutually transverse sides (T1, T2, T3) will cause a reaction that has the same intensity as the component directed toward the interior of the container, thereby imparting stability to said container along said vertical axis (X-X), - wherein said plurality of edges (C, C1, C2, C3) comprises: - at least two opposite edges (C1, C2) extending along a vertical axis (X-X), and - at least one edge transverse to said vertical axis (X-X) and opposite to said bearing edge, - wherein each of said at least two mutually transverse sides (T1, T3) is continuous with a respective one of said at least two opposite edges (C1, C2), and forms an intersection with the same angle as said taper angle (α) with each opposite edge, characterized in that it comprises a strip of stiffening material attached to said flexible sheet elements (2, 3), adjacent to the angled (α) intersection between said opposite edges (C1, C2) and said transverse sides (T1, T3) of the perimeter of said container. - a first flexible sheet element having a polygonal shape, which lies over a second flexible sheet element having a polygonal shape, said first and second flexible sheet elements being coupled together by welds formed along the perimeter of their polygonal shape, to thereby define a volume for containing said loose materials in liquid, pasty, granular or powder form, - said perimeter defining a plurality of edges (C, C1, C2 C3), one of said plurality of edges forming a bearing edge for the container to rest upon said base; - said bearing edge progressively reduces its transverse dimensions toward one end thereof with a taper angle (α) with respect to a vertical axis (X-X), such that, when the bearing edge rests on the base, one component of the weight force of the material contained in the container applied to said bearing edge causes a reaction that has the same intensity as said component directed toward the interior of the container, thereby causing said bearing edge to retract into said volume and imparting vertical stability to the container, - wherein said bearing edge comprises at least two mutually transverse sides (T1, T2, T3), which are inclined with a respective one of said taper angle (α) with respect to said vertical axis (X-X) of said container, such that, when said bearing edge rests on said base, the component of the weight force of the material contained in the container, applied to said at least two mutually transverse sides (T1, T2, T3) will cause a reaction that has the same intensity as the component directed toward the interior of the container, thereby imparting stability to said container along said vertical axis (X-X), - wherein said plurality of edges (C, C1, C2, C3) comprises: - at least two opposite edges (C1, C2) extending along a vertical axis (X-X), and - at least one edge transverse to said vertical axis (X-X) and opposite to said bearing edge, - wherein each of said at least two mutually transverse sides (T1, T3) is continuous with a respective one of said at least two opposite edges (C1, C2), and forms an intersection with the same angle as said taper angle (α) with each opposite edge, - wherein said plurality of edges (C, C1, C2, C3) comprises: - at least two opposite edges (C1, C2) extending along a vertical axis (X-X), and - at least one edge transverse to said vertical axis (X-X) and opposite to said bearing edge, - at least two opposite edges (C1, C2) extending along a vertical axis (X-X), and - at least one edge transverse to said vertical axis (X-X) and opposite to said bearing edge, - wherein each of said at least two mutually transverse sides (T1, T3) is continuous with a respective one of said at least two opposite edges (C1, C2), and forms an intersection with the same angle as said taper angle (α) with each opposite edge,