SMARTWOOL Intraknit™ fabric is 3D-knit with Merino Sport yarns to be lighter weight with a cleaner finish. The Women's Intraknit Thermal Merino Base Layer 1/4 Zip is perfect for a day of outdoor activity in cool to warm weather, because it will help keep your body dry and comfortable, and its fit won't slow you down. Only used once! Love it but I am a size S/M and this was too tight since it’s XS.