The paper by S. Stören and J. R. Rice discusses the onset of localized necking in thin sheets under biaxial stretching. They use a simplified constitutive model that applies the equations of deformation theory of rigid-plastic solids to fully-active stress increments, predicting the onset of localized necking. The predictions agree well with experimental observations, supporting the hypothesis that vertex formation on the yield locus under continued plastic flow is responsible for localized necking. This conclusion has implications for understanding ductile fracture in solids as a material instability. The authors also explore the role of initial imperfections and yield-vertex effects in localization instabilities. They derive conditions for strain localization and show that the equations of deformation theory can describe the destabilizing effect of a pointed vertex on the yield locus. Numerical results and experimental data are presented to support their findings, demonstrating that the predicted necking conditions align with observed 'forming limit' curves. The study highlights the sensitivity of localized necking to details in the stress-strain relations of the material and suggests that further efforts should be directed towards a more detailed description of work-hardening in solids.The paper by S. Stören and J. R. Rice discusses the onset of localized necking in thin sheets under biaxial stretching. They use a simplified constitutive model that applies the equations of deformation theory of rigid-plastic solids to fully-active stress increments, predicting the onset of localized necking. The predictions agree well with experimental observations, supporting the hypothesis that vertex formation on the yield locus under continued plastic flow is responsible for localized necking. This conclusion has implications for understanding ductile fracture in solids as a material instability. The authors also explore the role of initial imperfections and yield-vertex effects in localization instabilities. They derive conditions for strain localization and show that the equations of deformation theory can describe the destabilizing effect of a pointed vertex on the yield locus. Numerical results and experimental data are presented to support their findings, demonstrating that the predicted necking conditions align with observed 'forming limit' curves. The study highlights the sensitivity of localized necking to details in the stress-strain relations of the material and suggests that further efforts should be directed towards a more detailed description of work-hardening in solids.