The Rho pants now come in three weights the SL (superlight), LT (lightweight), and AR (expedition-weight), plus a wool variation on the LT, an shorter boot cut on all models. I own the LT and AR. Though they are pricy ($79 and $99, respectively), they were the only long underwear bottoms I could find in the US that featured a through-the-crotch zip. Since I am a devotee of the idea that you shouldn't have to undress or untie to answer nature's call, that was a must-have feature for me. I've worn these many times now, and I love them. They stretch, but hold their shape, and the fit is awesome. I got them on sale, but still, a lot of $$ for long underwear. I'll be wearing these for a long time.