  The establishment of the neutrino oscillations phenomenon as a solution to
both solar and atmospheric neutrino anomalies had two consequences: a new
oscillation mode, labelled $\mathbf{\theta_{13}}$, and the possibility to
observe CP violation, if $\mathbf{\theta_{13}}$ was sizeable. CP violation
implies that neutrino oscillations behave differently for neutrinos and
anti-neutrinos -- a rare fundamental phenomenon key for our understanding of
the Universe. The experimental demonstration of $\mathbf{\theta_{13}}$ has
aided the completion of a quest lasting half a century. The best
$\mathbf{\theta_{13}}$ knowledge is today inferred from high-precision reactor
neutrino disappearance. The Double Chooz (DC) experiment has played a
pioneering role in this channel by providing the first positive evidence, in
2011, in combination with the T2K experiment appearance data. The establishment
of $\mathbf{\theta_{13}}$ awaited the Daya Bay experiment's observation in
2012; confirmed soon after by the RENO experiment. Today's best knowledge on
$\mathbf{\theta_{13}}$ from reactor experiments is a key input to many neutrino
experiments. Here DC reports its first multi-detector $\mathbf{\theta_{13}}$
measurement exploiting several unprecedented techniques for a major precision
improvement.
