100% Merino - Soft merino fibers that regulates body temperature in all weather conditions and resists odors Gusseted underarms for mobility and comfort Offset shoulder seams to prevent friction Flatlock seams to prevent friction Backward set side seams to prevent friction. This was a go to base layer when I lived in the mountains. Also perfect as a base layer under my scrubs. This is lightly used. As I lost my other one! And now I don’t live in the mountains so someone else should have it!!