Forty years of fine-tuning sleeping bags got Rab to the Neutrino Pro 900 Sleeping Bag, one of its most advanced mountaineering bags to date, and not just because of its warmth--rated to negative twenty degrees, by the way. This bag's narrow mummy shape reduces pack size without sacrificing insulation, and the trapezoidal baffles keep the 800 fill power goose down lofted for maximum warmth and minimal heat loss.