Extensive measurements with poly(ethylene terephthalate) fibers of a broad range of orientational and crystalline order have been made and analyzed vis-a-vis two-phase (crystalline and noncrystalline) descriptions of solid polymer morphologies. The results show that no two fundamentally different methods provide the same estimates, or consistently even the same trends, of phase composition over a broad range of order. Modifications of the simple two-phase model by incorporating order-dependence of characteristic parameters, such as noncrystalline density, still lead to unsatisfactory results. No meaningful relationship has been found between such parameters and measures of order, such as calculated birefringence, the noncrystalline phase. However, because of the simplicity of two-phase models and the absence of any objective framework for correlating properties with measures of order with three or more phase compositions, continued use of two-phase models, with cognizance of their inherent limitations, is recommended in seeking directions for process and product development.