Built to be the only bag you need from -20 F to 40 F, we've updated our Backpacker Editors' Choice award-winning big-mountain bag, Sonic, for even greater warmth, tuneability, and reduced environmental impact. With this bag, we are launching our next-generation Thermo Gills™ with a new multistage zipper system that dramatically increases the usable temperature range of the bag and allows fine-tuning of comfort from within the bag. Sonic's new chevron baffle construction better compartmentalizes down, eliminating migration and keeping the down evenly lofted. Not to mention, it looks awesome.