Subtracting Roman Numerals
Date: 03/14/99 at 15:59:31 From: Heather Subject: Roman Numerals What are the rules for the "subtraction components" in writing Roman Numerals?
Date: 03/15/99 at 08:28:42 From: Doctor Rick Subject: Re: Roman Numerals These are the limits on the use of the subtraction method, according to the modern rules of Roman numerals: I. You can only subtract I, X, C, etc. (powers of 10; not V, L, or D). II. You can only subtract a single letter from a single numeral (no IIX or IXX). III. What you are subtracting cannot be any smaller than 1/10 of what you are subtracting it from. You can only subtract I from V or X, and X from L or C (MIM is not allowed). Here is a positive way to look at it. When converting an Arabic numeral to a Roman numeral, convert it one digit at a time. Write each piece as a Roman numeral, then stick them all together left to right. For instance, 1999 = 1000 + 900 + 90 + 9 = M + CM + XC + IX = MCMXCIX The rules make for longer numerals sometimes than you might make by breaking the rules. But they make numerals easier to read, because you can read the Arabic digits off in small groups (M CM XC IX) and the subtractions are easy. Even in ancient Rome, though the place-value system of Arabic numerals (and the zero necessary to make it work) had not been invented, people already thought in terms of decimal groups, the way an abacus works: each digit made up of a collection of ones and fives. In ancient times and even in the Renaissance, the rules were not very strict (any more than spelling rules were!). You could find examples that violate each of my rules. See these interesting sites: Roman Numerals: History and Use http://www.deadline.demon.co.uk/roman/intro.htm Roman Numeral Date Conversion Guide http://www2.inetdirect.net/~charta/Roman_numerals.html - Doctor Rick, The Math Forum http://mathforum.org/dr.math/
Search the Dr. Math Library:
Ask Dr. MathTM
© 1994- The Math Forum at NCTM. All rights reserved.