In contrast, in Archimedes' work the concept of centre of gravity is crucial.[7] On the Equilibrium of Planes I, which contains seven postulates and fifteen propositions, uses the centre of gravity for both commensurable and incommensurable magnitudes to justify the law of the lever, though some argue not satisfactorily.[2] Archimedes then proceeds to locate the centre of gravity of the parallelogram and the triangle, ending book one with a proof on the centre of gravity of the trapezium.