The REI Co-op Merino 185 Long-Sleeve Base Layer Top uses ultrafine 185 g merino wool for a soft, itch-free feel. Made of 100% merino wool, it naturally breathes, wicks sweat, dries quickly, and resists odors, suitable for multiday trips. It provides UPF 30 sun protection, flat seams and merrow stitch construction to prevent chafing, and underarm gussets for full movement. Its slim fit enhances performance. Center back length is 27 inches with an average weight of 4.7 oz. Machine washable.