  In this paper we derive a strong maximum principle for weak supersolutions of
nonlocal equations of the form $Iu=c(x) u$ in $\Omega$, where $\Omega\subset
\mathbb{R}^N$ is a domain, $c\in L^{\infty}(\Omega)$ and $I$ is an operator of
the form $Iu(x)=P.V.\int_{\mathbb{R}^N}(u(x)-u(y))j(x-y)\ dy$ with a
nonnegative kernel function $j$. We formulate minimal positivity assumptions on
$j$ corresponding to a class of operators which includes highly anisotropic
variants of the fractional Laplacian. Somewhat surprisingly, this problem leads
to the study of general lattices in $\mathbb{R}^N$. Our results extend to the
regional variant of the operator $I$ and, under weak additional assumptions,
also to the case of $x$-dependent kernel functions.
