The human visual system has much less acuity for spatial variation of colour than for brightness. Rather than conveying RGB, it is advantageous to convey luma in one channel, and colour information that has had luma removed in the two other channels. In an analogue system, the two colour channels can have less bandwidth, typically one-third that of luma. In a digital system each of the two colour channels can have considerably less data rate (or data capacity) than luma. Green dominates the luma channel; about 59% of the luma signal comprises green information. Therefore, it is sensible, and advantageous for signal-to-noise reasons, to base the two colour channels on blue and red. The simplest way to remove luma from each of these is to subtract it from the difference between a primary colour and luma. Hence, the basic video color-difference pair is B-Y, R-Y (pronounced “B minus Y, R minus Y”). The B-Y signal reaches its extreme values at blue (R = 0, G = 0, B = 1; Y = 0.114; B-Y = +0.886) and at yellow (R = 1, G = 1, B = 0; Y = 0.886; B-Y = –0.886). Similarly, the extreme of R-Y, ±0.701, occur at red and cyan. These are inconvenient values for both digital and analogue systems. The colour spaces YPbPr, YCbCr, PhotoYCC, and YUV are simply scaled versions of Y, B-Y, R-Y that place the extreme of the colour difference channels at more convenient values. The R-Y signal drives the vertical axis of the vectorscope.