  An observation space $\mathcal S$ is a family of probability distributions
$\langle P_i: i\in I \rangle$ sharing a common sample space $\Omega$ in a
consistent way. A \emph{grounding} for $\mathcal S$ is a signed probability
distribution $\mathcal P$ on $\Omega$ yielding the correct marginal
distribution $P_i$ for every $i$. A wide variety of quantum scenarios can be
formalized as observation spaces. We describe all groundings for a number of
quantum observation spaces. Our main technical result is a rigorous proof that
Wigner's distribution is the unique signed probability distribution yielding
the correct marginal distributions for position and momentum and all their
linear combinations.
