The Law of Definite Proportions states that a chemical compound always contains its component elements in fixed ratios by mass. This means that no matter how a compound is formed or where it is found, the proportion of each element remains constant. For example, in water (H2O), the ratio of hydrogen to oxygen is always 2:1 by mass. This law was first established by the chemist Joseph Proust in the early 19th century. It is fundamental to the field of chemistry, as it helps scientists understand the composition of substances and predict how they will react in chemical reactions.