The ratio of a to b is a way to convey the idea of relative magnitude of two amounts. Thus if the number a is always twice the number b, we can say that the ratio of a to b is "2 to 1." This ratio is sometimes written 2:1. Today, however, it is more common to write a ratio as a fraction, in this case 2/1.
At one time, ratios were in common use in solving problems and the terms "antecedent " for the numerator a and "consequent " for the denominator b were used. Today most problems concerning ratios are solved by treating ratios as fractions.