  In neutrino phenomenology, terms in the oscillation probabilities linear in
$\sin \theta_{13}$ lead naturally to the question ``How can one measure
$\theta_{13}$ including its sign?'' Here we demonstrate analytically and with a
simulation of neutrino data that ${\mathcal P}_{e\mu}$ and ${\mathcal
{P}_{\mu\mu}$ at $L/E = 2\pi/\Delta_{21}$ exhibit significant linear dependence
on $\theta_{13}$ in the limit of vacuum oscillations. Measurements at this
particular value of $L/E$ can thus determine not only $\theta_{13}$ but also
its sign, if CP violation is small.
