 In this connection it is worth mentioning a famous fallacious proof that all triangles are isosceles, which Klein made well known. One can show that this pseudo-proof violates the rules of the game of Euclid, extracting exact information from the diagram, which is never allowed. On the other hand, one should notice that Euclid’s postulates are not so much axioms in the Hilbertian sense, as rules of construction that are applied in the elaboration and handling of diagrams. Most of Euclid's postulates establish that certain constructions can be performed, the drawing of a line or a circle, the meeting of two lines under certain conditions (this last is Euclid's own formulation of postulate 5 “of the parallels”).  As for the central role of such rules of construction in Ancient geometry, it is interesting to remark that the Greeks said ‘diagramma’ to mean a mathematical proposition. 