When two elements combine with each other to form more than one compound, the weights of one element that combine with a fixed weight of the other are in a ratio of small whole number.
For example: There are five distinct oxides of nitrogen and the weight of oxygen in combination with 14 grams of nitrogen are in increasing order 8, 16, 24, 32 and 40 grams or in ratio of 1, 2, 3, 4, 5.
