  Hadronic jets are extremely abundant at the LHC, and testing QCD in various
corners of phase-space is important to understand backgrounds and some specific
signatures of new physics. In this article, various measurements aiming at
probing QCD in configurations where the theory modeling become challenging are
presented. Azimuthal angle de-correlations are sensitive to hard as well as
soft QCD emission, and in most of the events jets are produced in a
back-to-back configuration. Events where jets have a large rapidity separation
are also rare, and those without additional radiation between the jets are
exponentially suppressed. The modeling of radiation between very forward and
backward jets is complicated, and may require theoretical tools different with
respect to those normally used for central, high-pt events. Observables can be
created that are sensitive to all these effects, like the study of azimuthal
angle de-correlations between events where the two leading jets have large
rapidity separations. The two general-purpose detectors of the LHC have
measured these observables, and for some of them interesting deviations with
respect to the most commonly used theoretical models are observed.
