  Let $E \subset \Omega$ be a local almost-minimizer of the relative perimeter
in the open set $\Omega \subset \mathbb{R}^{n}$. We prove a free-boundary
monotonicity inequality for $E$ at a point $x\in \partial\Omega$, under a
geometric property called ``visibility'', that $\Omega$ is required to satisfy
in a neighborhood of $x$. Incidentally, the visibility property is satisfied by
a considerably large class of Lipschitz and possibly non-smooth domains. Then,
we prove the existence of the density of the relative perimeter of $E$ at $x$,
as well as the fact that any blow-up of $E$ at $x$ is necessarily a
perimeter-minimizing cone within the tangent cone to $\Omega$ at $x$.
