  A measurement is presented of the triple-differential dijet cross section at
a centre-of-mass energy of 8 TeV using 19.7 inverse femtobarns of data
collected with the CMS detector in proton-proton collisions at the LHC. The
cross section is measured as a function of the average transverse momentum,
half the rapidity separation, and the boost of the two leading jets in the
event. The cross section is corrected for detector effects and compared to
calculations in perturbative quantum chromodynamics at next-to-leading order
accuracy, complemented with electroweak and nonperturbative corrections. New
constraints on parton distribution functions are obtained and the inferred
value of the strong coupling constant is alpha[S](M[Z]) = 0.1199 +/- 0.0015
(exp) -0.0020 +0.0031 (theo), where M[Z] is the mass of the Z boson.
