A composite dowel for making an attachment between two plates (2, 3) provided with slot holes (4, 5) arranged one against another with their respective slot holes (4, 5) perpendicular to and facing one another forming a passage for said composite dowel, said respective slot holes (4, 5) perpendicular to and facing one another having the same width, so that said passage for the composite dowel has a square shape, said composite dowel comprising: - a retention dowel comprising a base, for the support of said retention dowel on one of said plates (2, 3), and an expandable oblong retention body intended for being housed in said slot holes (4, 5), said retention body being formed by four flexible tabs separated from one another by four through notches and extending from said base demarcating between them an inner passage, said base comprising a through hole aligned with said inner passage; and - a locking pin which is pressure-inserted into said retention dowel and comprises a head and a rod with four fins extending in two radial planes with respect to a central axis of said rod, said two radial planes being orthogonal to one another; said retention dowel and said locking pin being configured such that, in a final coupling position, said rod is pressure-introduced into said inner passage through said through hole and causes an elastic deformation of said tabs which radially expands said retention body, and said four fins are introduced into said four through notches; characterized in that: - said four tabs are made such that, in a free position of said retention dowel, in which said rod is not introduced into said inner passage, each of said tabs has an outer face in one plane, and said outer faces of the four tabs form an outer contour of said retention body in the form of a prismatic surface with a square cross-section in at least one segment of said retention body, wherein the vertices of said prismatic surface can be optionally beveled; - said four through notches are arranged at the vertices of said prismatic surface with a square cross-section, and said four through notches do not reach the ends of said tabs opposite said base, such that said ends of the four tabs are attached to one another; - said fins are arranged so that edges of said fins are arranged at the vertices of a square in at least one segment of said rod which, in said final coupling position, is introduced into said segment of the retention body having the outer contour in the form of a prismatic surface with a square cross-section. - a retention dowel comprising a base, for the support of said retention dowel on one of said plates (2, 3), and an expandable oblong retention body intended for being housed in said slot holes (4, 5), said retention body being formed by four flexible tabs separated from one another by four through notches and extending from said base demarcating between them an inner passage, said base comprising a through hole aligned with said inner passage; and - a locking pin which is pressure-inserted into said retention dowel and comprises a head and a rod with four fins extending in two radial planes with respect to a central axis of said rod, said two radial planes being orthogonal to one another; said retention dowel and said locking pin being configured such that, in a final coupling position, said rod is pressure-introduced into said inner passage through said through hole and causes an elastic deformation of said tabs which radially expands said retention body, and said four fins are introduced into said four through notches; characterized in that: - said four tabs are made such that, in a free position of said retention dowel, in which said rod is not introduced into said inner passage, each of said tabs has an outer face in one plane, and said outer faces of the four tabs form an outer contour of said retention body in the form of a prismatic surface with a square cross-section in at least one segment of said retention body, wherein the vertices of said prismatic surface can be optionally beveled; - said four through notches are arranged at the vertices of said prismatic surface with a square cross-section, and said four through notches do not reach the ends of said tabs opposite said base, such that said ends of the four tabs are attached to one another; - said fins are arranged so that edges of said fins are arranged at the vertices of a square in at least one segment of said rod which, in said final coupling position, is introduced into said segment of the retention body having the outer contour in the form of a prismatic surface with a square cross-section. - said four tabs are made such that, in a free position of said retention dowel, in which said rod is not introduced into said inner passage, each of said tabs has an outer face in one plane, and said outer faces of the four tabs form an outer contour of said retention body in the form of a prismatic surface with a square cross-section in at least one segment of said retention body, wherein the vertices of said prismatic surface can be optionally beveled; - said four through notches are arranged at the vertices of said prismatic surface with a square cross-section, and said four through notches do not reach the ends of said tabs opposite said base, such that said ends of the four tabs are attached to one another; - said fins are arranged so that edges of said fins are arranged at the vertices of a square in at least one segment of said rod which, in said final coupling position, is introduced into said segment of the retention body having the outer contour in the form of a prismatic surface with a square cross-section.