These are a special knit design that allows tremendous flexibility providing soft comfort as an under layer.  They are 41% Baby Alpaca 34% Acrylic 23% Nylon 2% Spandex.  They come in Men and Womens styles as a S/M and a Lg/XL for size choices.  These are a base layer for outdoor activities or comfort wear at home.  Get the Alpaca base layer sweater top for complete thermal coverage.