The discrete element method (DEM) is employed to explore the responses of three major versions of direct simple shear (DSS) apparatus, i.e., stacked-ring, Cambridge, and NGI types, during shearing. In this regard, results of a stacked-ring simple shear test on glass bead specimens are employed to validate the 3D-DEM model parameters. It is shown that different boundary conditions do not influence internal responses of the specimens, significantly. The uncertainties associated with the behavior of DSS specimens, such as stress and strain non-uniformity, principal stress and strain rotations, the orientation of the failure surface, anisotropy, and non-coaxiality, are investigated. Suggestions are presented to interpret the DSS test results and estimate the stress state inside the specimen during shearing using conventional boundary measurements in the laboratory testing. (C) 2016 Elsevier B.V. All rights reserved.