  We present cosmological results from the final galaxy clustering data set of
the Baryon Oscillation Spectroscopic Survey, part of the Sloan Digital Sky
Survey III. Our combined galaxy sample comprises 1.2 million massive galaxies
over an effective area of 9329 deg^2 and volume of 18.7 Gpc^3, divided into
three partially overlapping redshift slices centred at effective redshifts
0.38, 0.51, and 0.61. We measure the angular diameter distance DM and Hubble
parameter H from the baryon acoustic oscillation (BAO) method after applying
reconstruction to reduce non-linear effects on the BAO feature. Using the
anisotropic clustering of the pre-reconstruction density field, we measure the
product DM*H from the Alcock-Paczynski (AP) effect and the growth of structure,
quantified by f{\sigma}8(z), from redshift-space distortions (RSD). We combine
measurements presented in seven companion papers into a set of consensus values
and likelihoods, obtaining constraints that are tighter and more robust than
those from any one method. Combined with Planck 2015 cosmic microwave
background measurements, our distance scale measurements simultaneously imply
curvature {\Omega}_K =0.0003+/-0.0026 and a dark energy equation of state
parameter w = -1.01+/-0.06, in strong affirmation of the spatially flat cold
dark matter model with a cosmological constant ({\Lambda}CDM). Our RSD
measurements of f{\sigma}_8, at 6 per cent precision, are similarly consistent
with this model. When combined with supernova Ia data, we find H0 = 67.3+/-1.0
km/s/Mpc even for our most general dark energy model, in tension with some
direct measurements. Adding extra relativistic species as a degree of freedom
loosens the constraint only slightly, to H0 = 67.8+/-1.2 km/s/Mpc. Assuming
flat {\Lambda}CDM we find {\Omega}_m = 0.310+/-0.005 and H0 = 67.6+/-0.5
km/s/Mpc, and we find a 95% upper limit of 0.16 eV/c^2 on the neutrino mass
sum.
