The classic "Old Man's Shirt" as used by the US Army from World War 2 to the 2000s. These are of course newer production, manufactured mostly in the '80s or later. Prior to WWII, soldiers were not equipped with clothes that left a lot of room for improvement. Experiences in the war pushed towards progress and many armed forces discovered layered clothing and one key component of the system: a knit sweater. The US M-1943 High-Neck Sweater was such a success that it remained in use for several decades, being changed only slightly in details such as button size or exact color shade. Speaking of which, these are called OD but appear brown. These modern shirts are made of 100% Acrylic fibers. Good for allergic folks or if you want to avoid wool for other reasons. Acryl is a better alternative than wool-fanatics would have you believe. The shirt can be machine-washed with a wool cycle and tumble-dried at low heat. Five buttons, snug fit especially on the hem and sleeve cuffs. The knit is thinner compared to British woolly pullies, for example. This allows a wider range of uses because the key to staying warm is flexibility of the clothing system and avoiding wearing too much.