I read two very good reviews that guided me in my selection. I will echo those reviews hoping to provide a bit more clarity. I'm 5'11" (average weight) and typically always wear a Large. But the reviews guided me to order a Medium if I was going to wear these as an underlayer. The Medium does fit well as an underlayer, but if I were to reorder I would probably order a Large just for a bit more room. But I will reiterate, if I were ordering just for an underlayer, I would stick with the Medium. Great selection of colors.
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