For certain applications, it is more useful to define static friction in terms of the maximum angle before which one of the items will begin sliding. This is called the angle of friction or friction angle. It is defined as: tan ⁡ ⁡ θ θ = μ μ s and thus: θ θ = arctan ⁡ ⁡ μ μ s where θ θ is the angle from horizontal and μ s is the static coefficient of friction between the objects. This formula can also be used to calculate μ s from empirical measurements of the friction angle.