This work takes a simple phenomenological approach to the questions of when, how, and why a brittle polymer glass turns ductile and vice versa. Perceiving a polymer glass as a hybrid, we recognize that both the primary structure formed by van der Waals forces (network 1) and chain network (i.e., the vitrified entanglement network) (network 2) must be accounted for in any discussion of the mechanical responses. To show the benefit of this viewpoint, we first carried out well-defined melt-stretching experiments on four common polymer glasses (PS, PMMA, SAN, and PC) in a systematic way either at a fixed Hencky strain rate to a given degree of stretching at several temperatures or at a given temperature to different levels of stretching using the same Hencky rate. Then we attempted to preserve the effect of melt-stretching on the chain network structure by rapid thermal quenching. Subsequent room-temperature tensile extension of these melt-stretched amorphous polymers reveals something universal: (a) along the direction of the melt-stretching, the brittle glasses (PS, PMMA, and SAN) all become completely ductile; (b) perpendicular to the melt-stretching direction, the ductile glass (PC) becomes brittle at room temperature. We suggest that the transformations (from brittle to ductile or ductile to brittle) arise from either geometric condensation or dilation of load bearing strands in the chain network due to the melt-stretching. Regarding a polymer glass as a structural hybrid, we also explored two other cases where the ductile PC becomes brittle at room temperature: (1) upon aging near the glass transition temperature; (2) when blended with PC of sufficiently low molecular weight. These results indicate that (i) the strengthening of the primary structure by aging can raise the failure stress σ* to a level too high for the chain network to sustain and (ii) the PC blend becomes brittle upon weakening the chain network by dilution with short chains.