The aim of this short note is to give an alternative proof, which applies to functions of bounded variation
in arbitrary domains, of an inequality by Maz'ya that improves Friedrichs inequality. A remarkable feature
of such a proof is that it is rather elementary, if the basic background in the theory of functions of bounded
variation is assumed. Nevertheless, it allows to extend all the previously known versions of this fundamental
inequality to a completely general version. In fact the inequality presented here is optimal in several respects.
As already observed in previous proofs, the crucial step is to provide conditions under which a function of
bounded variation on a bounded open set, extended to zero outside, has bounded variation on the whole space.
We push such conditions to their limits. In fact, we give a sufficient and necessary condition if the open set
has a boundary with σ-finite surface measure and a sufficient condition if the open set is fully arbitrary. Via a
counterexample we show that such a general sufficient condition is sharp.
