the Rolls Royce RB211 is triple-spooled. basically it means the engine has 3 different pressure systems, which is comprised of the compressor and the turbine. the three pressure systems (high pressure, intermediate pressure and low pressure) systems are independent of each other (they have independent shafts) and are free to rotate at their own optimum speed (the N1 fan must rotate slower than the internal compressors). as a result, the efficiency of the engine increases.