  We present foreground-reduced CMB maps derived from the full Planck data set
in both temperature and polarization. Compared to the corresponding Planck 2013
temperature sky maps, the total data volume is larger by a factor of 3.2 for
frequencies between 30 and 70 GHz, and by 1.9 for frequencies between 100 and
857 GHz. In addition, systematic errors in the forms of
temperature-to-polarization leakage, analogue-to-digital conversion
uncertainties, and very long time constant errors have been dramatically
reduced, to the extent that the cosmological polarization signal may now be
robustly recovered on angular scales $\ell\gtrsim40$. On the very largest
scales, instrumental systematic residuals are still non-negligible compared to
the expected cosmological signal, and modes with $\ell < 20$ are accordingly
suppressed in the current polarization maps by high-pass filtering. As in 2013,
four different CMB component separation algorithms are applied to these
observations, providing a measure of stability with respect to algorithmic and
modelling choices. The resulting polarization maps have rms instrumental noise
ranging between 0.21 and 0.27$\,\mu\textrm{K}$ averaged over 55 arcmin pixels,
and between 4.5 and 6.1$\,\mu\textrm{K}$ averaged over 3.4 arcmin pixels. The
cosmological parameters derived from the analysis of temperature power spectra
are in agreement at the $1\sigma$ level with the Planck 2015 likelihood.
Unresolved mismatches between the noise properties of the data and simulations
prevent a satisfactory description of the higher-order statistical properties
of the polarization maps. Thus, the primary applications of these polarization
maps are those that do not require massive simulations for accurate estimation
of uncertainties, for instance estimation of cross-spectra and
cross-correlations, or stacking analyses.
