Diamond and simplex models are useful in dealing with systems under parameter perturbations. A diamond (simplex) stability region 𝒟 is an open set in the complex plane with the property that if the zeros of all vertex polynomials of any diamond (simplex) family are all in 𝒟, then the zeros of all the polynomials of this family are all in 𝒟. It is shown that some typical stability regions used in control systems design are diamond (simplex) stability regions. Some discussions and extensions are also presented.