Example map constructed for nulling CMB anisotropies by differencing the 100 GHz and 70 GHz channels. A good fraction of the Galactic emission which stands out at low latitudes arises from CO in the 100 GHz channel (see Sect. 8.2.4). The overall impression of green, a colour not used in the colour bar, is due to the interaction between noise, the colour scale, and the display resolution. Positive and negative swings between pixels in the 70 GHz noise map pick up reds and blues “far” from zero, which when displayed at less than full-pixel resolution give green.