The law of reciprocal proportion states that,
“When two different elements separately combine with a fixed mass of a third element, the proportion in which they combine with one another shall be either in the same ratio or some multiple of it.”
For example, For example, sulphur and carbon both form compounds with hydrogen. In methane 12 g of carbon react with 4 g of hydrogen. In hydrogen sulphide, 32 g of sulphur react with 2 g of hydrogen (i.e. 64 g of S for 4 g of hydrogen). Sulphur and carbon form a compound in which the C: S ratio is 12:64 (i.e. CS2).