Glasses exist in three generic elastic phases: flexible, intermediate and stressed-rigid, which are determined by the connectivity of their backbones. Measurements of glass transition temperatures (T g s) using modulated-differential scanning calorimetry permits distinguishing these phases by their characteristic non-reversing enthalpies (ΔH nr ) at T g s. In Raman scattering, characteristic elastic power-laws are observed in intermediate and stressed-rigid phases. Liquid fragilities are found

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... o correlate with ΔH nr terms in covalent networks but not in modified oxide or H-bonded networks. In the latter systems weak network links exist, which cease to constrain networks as the temperature T > T g and viscosities plummet. Intermediate phase glasses are composed of rigid but unstressed networks that are in a state of quasi-equilibrium and age minimally. Such glasses usually form space filling networks and are structurally self-organized.