Castelli Flanders Base Layer LS Multidenier 100% polyester 3D-structure rib-knit main body for mechanical wicking and exceptional fi Perforated 100% polyester mesh under arms for additional cooling Multidenier 100% polyester 4-way stretch sleeves for great wicking and fit 4-needle flat-lock stitching lies flat for total comfort Mock turtle neck on long-sleeve model, Flat finish bottom, Longer back stays tucked in