RFL stands for Real-F’in-Light. They are woven thin-to-win, perfect for those performance race fits. The socks are part of Darn Tough’s Vertical series, which is designed for up and downhill skiing. The RFLs offer a superior fit within a ski boot liner and are breathable for long uphill slogs. They are also antimicrobial so you can use them for many days without noticeable odor due to bacteria build-up. And of course, they feature Darn Tough’s legendary toughness and lifetime guarantee. I don’t think we need to mention they are ultralight, since the name explains that rather forcefully.
Update 2018/19: The beloved RFL sock received a few updates beyond the obvious cosmetic facelift of the new Black / Green colorway. The instep panel has been tightened for better heel retention and the forefoot has been loosened just a touch to reduce toe fatigue and allow a more relaxed toe splay. Bottom line is that we didn’t think they could get more comfortable, but they did.