The Tresca yield criterion is classical and important to the theory of plasticity. It is usually formulated in terms of the difference between the maximum and minimum principal stresses. Difficulties have been encountered in attempts to explicitly express the plastic normality-flow rule associated to it, because the difference between the maximum and minimum principal stresses is generally not differentiable but subdifferentiable with respect to the stress tensor. In this work, the corresponding subdifferential is determined and specified for all possible cases; the explicit mathematical expressions are obtained for the plastic normality-flow rule relative to the Tresca yield criterion.