 MPEG-1 operates on video in a series of 8×8 blocks for quantization.However, to reduce the bit rate needed for motion vectors and because chroma (color) is subsampled by a factor of 4, each pair of (red and blue) chroma blocks corresponds to 4 different luma blocks.This set of 6 blocks, with a resolution of 16×16, is processed together and called a "macroblock".