# Proportionality

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 ${\displaystyle x\propto y}$.[1] The fixed number of a proportionality relationship is called the constant of proportionality.[2][3]