  We present cosmological parameter results from the final full-mission Planck
measurements of the CMB anisotropies. We find good consistency with the
standard spatially-flat 6-parameter $\Lambda$CDM cosmology having a power-law
spectrum of adiabatic scalar perturbations (denoted "base $\Lambda$CDM" in this
paper), from polarization, temperature, and lensing, separately and in
combination. A combined analysis gives dark matter density $\Omega_c h^2 =
0.120\pm 0.001$, baryon density $\Omega_b h^2 = 0.0224\pm 0.0001$, scalar
spectral index $n_s = 0.965\pm 0.004$, and optical depth $\tau = 0.054\pm
0.007$ (in this abstract we quote $68\,\%$ confidence regions on measured
parameters and $95\,\%$ on upper limits). The angular acoustic scale is
measured to $0.03\,\%$ precision, with $100\theta_*=1.0411\pm 0.0003$. These
results are only weakly dependent on the cosmological model and remain stable,
with somewhat increased errors, in many commonly considered extensions.
Assuming the base-$\Lambda$CDM cosmology, the inferred late-Universe parameters
are: Hubble constant $H_0 = (67.4\pm 0.5)$km/s/Mpc; matter density parameter
$\Omega_m = 0.315\pm 0.007$; and matter fluctuation amplitude $\sigma_8 =
0.811\pm 0.006$. We find no compelling evidence for extensions to the
base-$\Lambda$CDM model. Combining with BAO we constrain the effective extra
relativistic degrees of freedom to be $N_{\rm eff} = 2.99\pm 0.17$, and the
neutrino mass is tightly constrained to $\sum m_\nu< 0.12$eV. The CMB spectra
continue to prefer higher lensing amplitudes than predicted in base
-$\Lambda$CDM at over $2\,\sigma$, which pulls some parameters that affect the
lensing amplitude away from the base-$\Lambda$CDM model; however, this is not
supported by the lensing reconstruction or (in models that also change the
background geometry) BAO data. (Abridged)
