Law of multiple proportions

In chemistry, one of the basic laws of stoichiometry, alongside the law of definite proportions.

Two elements which form more than one compound between them will have compounds whose mass ratio between the two elements are related to each other as a ratio of small whole numbers.

For example, considering two of the carbon oxides: CO and CO₂.

In the first, the ratio is 1/1 and in the second 1/2. The ratio between these ratios is a ratio between small whole numbers, or

     1/1 : 1/2  = 2:1