B − z vs. z − K plot of all GMASS objects, showing which objects were included in the mask for spectroscopy. Colours indicate photometric redshift either above z = 1.4 (red) or below z = 1.4 (blue). Objects included for spectroscopy are marked by crosses (no redshift obtained), boxes (tentative redshift obtained), and circles (secure redshift obtained). Note that this includes mask fillers (at low redshift) and stars (for monitoring of the seeing and telescope pointing). The limits of the BzK selection are indicated by dashed lines.