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The mechanical properties and electronic structure of polymorphic tungsten carbides with different stacking sequences have been investigated by means of density functional theory. The stacking sequence strongly influences the stability and mechanical strength of these polymorphs. Using the plastic parameters based on resistance to slip, we found that the newly identified metastable hP4[194] structure (expressed by the Pearson symbol and a space group number) possesses the highest plastic resistance despite its lower elastic moduli as compared to the thermodynamically stable hP2[187]. The cF8[225] structure is thermodynamically and dynamically unstable at 0 K, but it shows anharmonic stabilization at finite temperature, in agreement with the experimental data. Our results provide a way to increase material strength and hardness by varying the crystal lattice stacking sequence. [ABSTRACT FROM AUTHOR]