The Arc'teryx Eon SLW T-Shirt is a super light technical baselayer made with naturally soft & odour resistant Merino wool. Features Next-to-skin fit MAPP Eon Mountain Merino wool Flatlocked seams for added comfort Anatomical shaping & gusseted underarms for increased mobility WEIGHT: 168g (size M). Glossary MAPP: Merino Advanced Performance Program Uses Merino wool's natural properties to create a comfortable personal environment. Merino wool absorbs moisture, moving it away from the skin & produces a small amount of heat. Eon Mountain Merino wool:A double-faced fabric with added spandex that is warm even when wet, keeps its shape, & reacts with changing humidity levels to regulate body temperature. It is also highly resilient, itch-free & ultra quick-drying.