The Multi Resolution Clean (MRC, Wakker & Schwarz, 1988) is the first try to introduce the notion of cleaning at different scales. MRC works on two intermediate maps (strictly speaking MRC is a double-resolution CLEAN algorithm). The first map is a smoothed version of the dirty map and the second map, called difference map, is obtain by subtraction of the smoothed map from the original dirty map. Since the measurement equation is linear, both maps can be cleaned independently (using a smoothed and a difference dirty beam, respectively). The underlying idea is that extended sources in the dirty map will look like more "point-like" with respect to the smoothed dirty beam in the smoothed map. MRC is faster than the basic CLEAN algorithms because fewer clean components are needed to reproduce an extended source feature in the smoothed map than in the original map.