The redshift origins of the background at 850 μm and 1.16 mm under various scenarios (see Section 4 for details). The different plotting styles indicate different scenarios; all magenta points/bars are for the 850 μm background, while all black points/bars are for the 1.16 mm background. Points/bars are offset within the redshift bins for clarity. In what we deem the most likely scenario, 60–88 per cent of the 1.16 mm background comes from galaxies at z > 1.3.