This paper deals with a class of conservative nonlinear oscillators of the formẍ(t) + f (x(t)) = 0, where f (x) is analytic. A transformation of time from t to a new time coordinate τ is defined such that periodic solutions can be expressed in the form x(τ ) = A 0 + A 1 cos 2τ . We refer to this process of trigonometric simplification as trigonometrification. Application is given to the stability of nonlinear normal modes (NNMs) in two-degree-of-freedom systems.