Bayer Filter
The dominant colour filter array pattern, using two green filters for every red and blue in a repeating square of four.
Formula / rule
RGGB in 2×2 blocks
In depth
Green is doubled because human vision derives most of its luminance information from the green part of the spectrum, so weighting the sampling toward green maximises perceived sharpness. The regular pattern is also the reason Bayer sensors are prone to moiré on fine repeating detail, which the optical low-pass filter was invented to suppress.
Examples in practice
Half of a Bayer sensor's photosites record green, a quarter red and a quarter blue.