The jersey knit fabric level-1 of the E.C.W.C.S. System helps provide extra warmth in cold climates. Rothco's silk weight underwear top is highly breathable, with moisture-wicking properties and features anti-microbial & anti-odor properties which protect against bacteria and bad odors from forming. The level-1 underwear top also has a long back tail and thumb-hole sleeves. It's great for layering in the wintertime and works well with our other ECWCS items or underneath t-shirts, jackets and more.  Rothco Gen III Level I Silk Weight Underwear Tops are part of the ECWCS (Extended Cold Weather Clothing System) which provides extra warmth in cold climates.