Very convenient and comfortable thermal. The company refers to it as a thermal underwear and they are probably right. It is not the classic thermal that we know, but it has a very special weave (hence the name), it is EXTREMELY stretchy (Pentagon offers it in 2 sizes: a. XS to M and b. L to XXL) and very soft. It does its job very well, it holds the body tightly and retains heat inside. Sweat is easily expelled. It is NOT recommended for extreme temperatures, as in that case it would be good to look at the corresponding Columbia ones, with the internal membrane, which of course cost almost three times as much. The short-sleeved one may be more convenient, but I imagine it won't retain the same amount of heat. Once again, Pentagon does not disappoint us.