Law of Multiple Proportions
When one element can combine with another to form more than one compound, the mass ratios of the elements in the compounds are simple whole-number ratios of each other.
Example: in CO and in CO2, the oxygen-to-carbon ratios are 16:12 and 32:12, respectively. Note that the second ratio is exactly twice the first, because there are exactly twice as many oxygens in CO2 per carbon as there are in CO.
See also: Law of Conservation of Mass.