比例式
A 1 : A 2 : ⋯ : A n = X 1 : X 2 : ⋯ : X n ⟺ A 1 X 1 = A 2 X 2 = ⋯ = A n X n {\displaystyle A_{1}:A_{2}:\cdots :A_{n}=X_{1}:X_{2}:\cdots :X_{n}\iff {A_{1} \over X_{1}}={A_{2} \over X_{2}}=\cdots ={A_{n} \over X_{n}}}
と定義され、比の場合と同様に等式
A 1 : A 2 : ⋯ : A n = X 1 : X 2 : ⋯ : X n {\displaystyle A_{1}:A_{2}:\cdots :A_{n}=X_{1}:X_{2}:\cdots :X_{n}}
または
A 1 X 1 = A 2 X 2 = ⋯ = A n X n {\displaystyle {A_{1} \over X_{1}}={A_{2} \over X_{2}}=\cdots ={A_{n} \over X_{n}}}
のことを比例式とよぶ。