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Roman numerals are a numeral system that originated in ancient Rome and remained the usual way of writing numbers throughout Europe well into the Late Middle Ages. Numbers in this system are represented by combinations of letters from the Latin alphabet. Modern usage employs 7 symbols, each with a fixed integer value:

I = 1, V = 5, L = 50, C = 100 and M = 1000.

- Repeating a numeral up to 3 times represents addition of the number. For example, III represents 1 + 1 + 1 = 3.
- Only I, X, C, and M can be repeated. V, L, and D cannot be.
- Writing numerals that decrease from LEFT to RIGHT represents addition of the numbers. Eg: LXI represents 50 + 10 + 1= 61 and MCVI represents 1000 + 100 + 5 + 1 = 1106.
- A subtraction rule. Writing a smaller numeral to the left of a larger numeral represents subtraction. For example, IV represents 5 - 1 = 4 and IX represents 10 - 1 = 9. To avoid ambiguity, the only pairs of numerals that use this subtraction rule are:

IV = 4 (5-1)

IX = 9 (10-1)

XL = 40 (50-10)

XC = 90 (100-10)

CD = 400 (500-100)

CM = 900 (1000-100)

- To represent larger numbers, a bar over a numeral means to multiply the number by 1000.

For example:

D represents 1000 x 500 = 500,000.

M represents 1000 x 1000 = 1,000,000.

XXIII represents 23 x 1000 = 23,000.

CCCCC represents 500 x 1000 = 500,000.

- Zero & decimal. The Roman numeral system did not have zero or decimal, and Romans had no concept of it in their arithmetic. Which is one reason why Roman numerals are so clumsy for calculation.