Mask for our analysis of the Planck-Hi correlation. The coloured area that is not blue defines the data used to compute the correlation measures. Within this area, the median N_HI is 2.1 × 10²⁰ H cm^-2, and N_HI < 3 × 10²⁰ H cm^-2 for 74% of the pixels. The blue patches correspond to regions where the absolute value of the residual emission is higher than 3σ at 857 GHz (Fig. 4). The circular hole near the Southern Galactic pole corresponds to the area where Hi gas in the Galaxy cannot be well separated because the mean radial velocity of the gas in the MS is within the Galactic range of velocities.