Smart Compression is technical fabric that conforms to your anatomy like an Intelligent Second Skin. Compression means that it should fit closely but comfortably to the body to activate the PostureCueTM. Most people can wear two sizes of our shirts. It is a matter of personal comfort and how much PostureCueTM you want. The more form fitting the more PostureCueTM. If you like a little less, than opt for the larger siz