 Technical divers have generally used a redundant gas supply system, either isolation-manifolded or independent back-mounted cylinders.Recreational divers have traditionally resorted to using buddy supplied gas, or relatively small bailout ‘pony cylinders’ or ‘ascent bottles’ for out-of-air emergencies.Whether attached to the primary cylinder, or slung at the chest, these cylinders often present problems with stability and streamlining, whilst not always providing more than a bare minimum supply of air for emergency ascent.