The law of definite proportions states that a chemical compound always has exactly the same proportion of elements by mass. Joseph Proust, a French chemist, first observed it in 1794. For this reason, it is sometimes called Proust's law.
Amounting to the same thing is the law of constant composition. This states that all samples of a given chemical compound have the same elemental composition by mass. For example, oxygen makes up about 8/9 of the mass of any sample of pure water, while hydrogen makes up the remaining 1/9 of the mass. The law of definite proportions is the basis of stoichiometry, together with the law of multiple proportions.