Artistic Lineage Mediums in order of lean to fat:
1) Lean Painting Medium - Useful to tone a panel or canvas, for underpainting and sketching, for initial passages of lean paint, and as an initial "couch." A "couch" is a thin layer applied onto the canvas to allow subsequent brushwork to flow effortlessly.
2) Fat Painting Medium - Useful for thinning paint for staining and glazing effects which has a leveling effect showing minimal (if any) brushstrokes. Also, useful for "oiling out". It can also be used as a "couch" applied over previously dried brushwork. A scant medium application allows paint passages in the middle stages of the painting to glide and blend easier.
3) Painting Butter - Useful for extending oil colors while retaining their viscosity. It allows for impasto and sgraffito mark making with the knife, brush, and other tools. It also gives of lubrication to the loaded brush across the panel or canvas.