In chemistry, the law of definite proportions, sometimes called Proust's law or the law of definite composition, states that a chemical compound always contains exactly the same proportion of elements by mass. An equivalent statement is the law of constant composition, which states that all samples of a given chemical compound have the same elemental composition by mass. For example, oxygen makes up about /₉ of the mass of any sample of pure water, while hydrogen makes up the remaining /₉ of the mass. Along with the law of multiple proportions, the law of definite proportions forms the basis of stoichiometry.