Interestingly, commercial poster sites in the UK are still based on the Crown paper size of 15 x 20". They are described as "sheets" 1 sheet being Double crown: 20 x 30"; 2 sheet Quad crown 30 x40" and so on.
Fine stationery, mostly for wedding sets these days, still uses the old size and finish designations. But all good; something has to justify the inflated pricing. 🙂
There are a lot of classic measurement systems across modern history, but paper and its endless systems of weight, size, finish etc. remains in a class of its own. 🙂