Plus fours are so called because their length is to the knee plus four inches. Hence there are also plus twos and plus sixes. Not sure about minus numbers though!
After watching an 'episode' of The Map Reading Company' about a VERY expensive; £500, prismatic compass on the market, realised that my dad's army issue prismatic compass which he was issued with in 1940 had two 'features' which I'd been totally unaware of for over fifty years - !
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