In general anaesthesia, recovery and emergency resuscitation procedures involve the use of a breathing system. The principle of anaesthetic systems is to efficiently eliminate exhaled carbon dioxide, without greatly increasing dead space or resistance. In 1954 Mapleson classified breathing systems based on their efficiency in eliminating carbon dioxide during spontaneous respiration. The breathing systems are classified in order of fresh gas flow (FGF) to prevent rebreathing during spontaneous respiration. Further information on the Mapleson classification can be found in our educational section.