What are the rules and regulations of roman numeral when to add and to substract?

Answer 1

The rules and regulations now governing the Roman numeral system had nothing to do with the ancient Romans because they were introduced during the Middle Ages presumably to make Roman numerals more compatible and easily converted into Hindu-Arabic numerals that were gradually superseding Roman numerals at the time but in general when a smaller numeral follows a larger numeral they are added as for example LX = 50+10 => 60 and when a smaller numeral precedes a larger numeral they are subtracted as for example XL = -10+50 => 40

Under today's rules 1999 = MCMXCIX meaning 1000-100+1000-10+100-1+10

But the Romans would have worked it out as MDCCCCLXXXXVIIII and simplified it to IMM meaning -1+2000 = 1999

Answer 2

In Roman numerals, certain letters are used to represent specific values. When a smaller numeral appears before a larger one, it is subtracted from the larger one. For example, IV represents 4 (5-1) and IX represents 9 (10-1). However, when a smaller numeral appears after a larger one, it is added to the larger one. For instance, VI represents 6 (5+1) and XI represents 11 (10+1).