Our sizing is dual for our knit items (S/M, M/L, L/XL, XL/XXL) because we use a fabric called Roma Ponte. Roma Ponte is a double knit fabric that has some give to it, allowing a little more room for the wearer. We find that this system works well for our customers. Please reference our size chart for measurement information, and feel free to send us a message if you have questions! The size chart applies to both our knit items and our Trenches.