A proportionality relationship happens when two quantities or numbers "x" and "y" are related multiplicatively by a fixed number. This can occur when either their ratio "x"/"y" is a fixed number (direct proportionality), or their product "xy" is a fixed number (inverse proportionality). If "x" is directly proportional to "y", then we write formula_1. The fixed number of a proportionality relationship is called the constant of proportionality.