Despite being the middle child of Arc'teryx baselayers, the Women's Long Sleeve Phase AR Zip Neck Top deserves respect. This mid weight baselayer fits like a second skin, so you can pile on the layers or wear it alone for a sleek, aerodynamic profile. The combination of mid and light weight technical fabrics creates an efficient system to keep you warm and still provide escape routes for excess heat and sweat. Underarm gussets mean you can do the wave at football games or stretch for a tricky hold and not rip your shirt. Flatlock stitching reduces the likelihood of the seam induced chafing. The Phase AR Zip Neck might be the perfect top for your fall backpacking trip: wear it alone during the day as you're hiking towards camp, throw on a fleece while you're setting up the tent, and then add a shell when darkness falls and daytime heat seems imagined.