Two features characterise the treatment of Geometry as presented in Euclid's “Elements”: (1) the propositions are arranged in a definite sequence which cannot be greatly altered without invalidating the proofs; (2) there are no methods of proof applicable to a large number of propositions. If we except the method of reductio ad absurdum, it is scarcely an exaggeration to say, for example, that in Book I no three propositions are proved by the same method.