We use a Next Level 3600 100% ringspun cotton shirt. Because it is a 100% cotton shirt, it will shrink a little but not an entire size. You can keep shrinkage to a minimum by wash cold/hang dry. This is an athletic fit shirt therefore, if you're in-between sizes we recommend going with the next size up. if you have any questions or concerns please email or call us. Support@westerngritco.com or (909) 687-7019