  A measurement for inclusive 2- and 3-jet events of the azimuthal correlation
between the two jets with the largest transverse momenta, $\Delta\phi_{12}$ ,
is presented. The measurement considers events where the two leading jets are
nearly collinear ("back-to-back") in the transverse plane and is performed for
several ranges of the leading jet transverse momentum. Proton-proton collision
data collected with the CMS experiment at a center-of-mass energy of 13 TeV and
corresponding to an integrated luminosity of 35.9 fb$^{-1}$ are used.
Predictions based on calculations using matrix elements at leading-order and
next-to-leading-order accuracy in perturbative quantum chromodynamics
supplemented with leading-log parton showers and hadronization are generally in
agreement with the measurements. Discrepancies between the measurement and
theoretical predictions are as large as 15%, mainly in the region 177$^\circ$
$<$ $\Delta\phi_{12}$ $<$ 180$^\circ$. The 2- and 3-jet measurements are not
simultaneously described by any of models.
