Joseph Proust's Law of Definite Proportions state that each compound is composed of exactly the same proportion of elements by mass. This law is independent of how much of the compound is used. For example, water is composed of hydrogen and water. If you take the mass percentages of the two elements, hydrogen will always be 1/9 of water's mass, while oxygen will always be 8/9 of water's mass.
This law helped shape Dalton's work on the Law of Multiple Proportions which states that several compounds can contain the same group of elements. The ratios of the elements' masses in the compounds are always whole numbers.
