Roman Numerals were a numbering system used on ancient Earth, and later on Safehold. The numbers from one to ten can be displayed as such I, II, III, IV, V, VI, VII, VIII, IX, X. The Roman Nuneral system was derived from the Latin Alphabet and were used by the Roman Empire, and afterwards until the 14th century AD, when they were replaced with the Arabic Numerals.
Although based upon the decimal system, meaning it used the number ten as its base, this system lacked a representation of zero, making advanced math quite difficult. For that reason it was selected for use on Safehold. One of the first of several introduced technologies and theories by Merlin Athrawes were Arabic numerals, which quickly spread from Charis to the rest of Safehold. (OAR)