How to Read Roman Numerals
Introduction
While 21st century numerals are relatively easy, the same cannot be said about those at the tail end of the previous century. At the end of a twentieth century movie, on a clock face, in a book preface, or on the cornerstone of a building you may encounter complicated dates. Here are some tips to show you how to read modern Roman Numerals.
Difficulty Level: average
Here's How:

Learn the meanings of the letters: M=1000, D=500, C=100, L=50, X=10, V=5, and I=1.

The letters (Roman numerals) are arranged from left to right in descending order of valuation and are simply added to each other (MM = 1000 + 1000 = 2000 and MDCLXVI = 1000 + 500 + 100 + 50 + 10 + 5 + 1 = 1666), but...

Sometimes there is a lower value numeral in front of (to the left of) a higher value numeral to indicate that the lower value should be subtracted from the adjacent higher value (XL = ten taken from fifty = 40 or (IX = one taken from ten = 9)).

This subtraction socalled rule is particularly useful in modern notations where we avoid four or more identical, consecutive numerals. Thus, instead of writing IIII, we write IV. The Romans preferred IIII. Today we think of IIII as wrong.

There are a few more rules or rather nonstandardized styles that apply to ancient, but not modern Roman numerals. "9" could be written "VIIII."
A smaller numeral to the left of a larger could mean multiplication (VM = 5000).
 Occasionally, two smaller numerals to the left are to be subtracted from the sum of those to the right (IIXX = 18).
 An oversized terminal I could mean the "1" should be doubled (MCMVI = 1907).
Tips:
 In modern usage, the smaller value to be subtracted from the larger is limited in usage. It can only be one Roman Numeral unit smaller than the numeral to the right. Thus, instead of writing "IMM" for "1999," we have to read/translate the cumbersome MCMXCIX (1000 + [1000  100] + [100  10] + [10  1]). But then, we also use the less difficult to read XL instead of XXXX, so it would be unfair to think modern teachers of Roman numerals want us to do it the hard way. They are just striving for consistency.
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