When two elements combine to form two or more chemical compounds, then the mass of one of the compounds in a fixed mass of the other holds a simple measure of each other is the law of equality.

Example: Carbon combines oxygen to form two combinations of carbon dioxide and carbon monoxide

The mass of oxygen that combines with the mass of carbon in carbon dioxide and carbon monoxide 32 and 16. So oxygen bear: a ratio of 32:16 or 2: 1

Example: Sulfur combines with oxygen to form sulfur trioxide and sulfur dioxide

The amount of oxygen mixed with the mass of sulfur determined in SO3 and SO2 is 48 and 32. So oxygen carries a ratio of either 48:32 or 3: 2