Everyone knows that a dozen may be either twelve or thirteen, a score either twenty or twenty-one, a hundred either one hundred or one hundred and twenty, and a thousand either one thousand or one thousand two hundred. The higher numbers are the old Teutonic computations. Hickes tells us that the Norwegians and Icelandic people have two sorts of decad, the lesser and the greater called “Tolfræd.” The lesser thousand = 10 x 100, but the greater thousand = 12 x 100. The word tolf, equal to tolv, is our twelve. (Institutiones Grammaticæ, p. 43.)
Five score of men, money, or pins, Six score of all other things.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Thousand from Infoplease: