Element (X) forms different compounds with Element (Y)
Ex:
Compound 1: XY6 (2.82g)
Compound 2: XY? (1.41g)
Compound 3: XY? (0.94g)
Compound 1: 2.82/6 = 0.47
Compound 2: 1.41/0.47 = 2
Compound 3: 0.94/0.47 = 3