The operator form of the constitutive equation containing fractional derivatives leads to an expression for the complex modulus which is a ratio of polynomials of fractional order in reduced frequency. A ratio of factored polynomials is developed by use of Bode diagrams; another related form arises from the generalized fractional Maxwell model. Bode diagrams are used to determine parameter values. Interconversion to other mechanical properties is outlined. The results potentially form the basis of a new theory.