  Let $A,B \subseteq \mathbb{R}^d $ both span $\mathbb{R}^d$ such that $\langle
a, b \rangle \in \{0,1\}$ holds for all $a \in A$, $b \in B$. We show that $
|A| \cdot |B| \le (d+1) 2^d $. This allows us to settle a conjecture by Bohn,
Faenza, Fiorini, Fisikopoulos, Macchia, and Pashkovich (2015) concerning
2-level polytopes. Such polytopes have the property that for every
facet-defining hyperplane $H$ there is a parallel hyperplane $H'$ such that $H
\cup H'$ contain all vertices. The authors conjectured that for every
$d$-dimensional 2-level polytope $P$ the product of the number of vertices of
$P$ and the number of facets of $P$ is at most $d 2^{d+1}$, which we show to be
true.
