There exists a constant $C>0$ such that every Lipschitz continuous function $\varphi:\tau\to\mathbf{R}$ satisfies

$$C^{-1}\lip_{\tau}(\varphi)\leq\sup_{e\in\mathcal{E}(\tau),v\in A}|D_{v}\varphi(e)|\leq C\lip_{\tau}(\varphi)$$

where $A\subset\mathrm{int}(\tau)$ denotes the set of points at which $\varphi$ is differentiable.