This is a list of Rolls-Royce Merlin variants. Engines of a similar power output were typically assigned different model numbers based on supercharger or propeller gear ratios, differences in cooling system or carburettors, engine block construction, starting system, or arrangement of engine controls. All Merlin engines except versions 131 and 135 were "right hand tractor", i.e. the propeller rotated clockwise viewed from behind.