Impact's driver underwear is made of fire-retardant Nomex and comes in a variety of options including: long-sleeve and short-sleeve tops, bottoms, socks and either single-eye or dual-eye balaclavas. They are all extremely durable and moisture-managing and have superior thermal performance and are soft, comfortable and breathable. Works perfectly under our firesuits and is strongly recommended for use by use even if your sanctioning body doesn't require it, your safety should always be the highest level possible. One size fits all.