Roman numerals are a numeral system that originated in ancient Rome and were used throughout the Roman Empire. They use combinations of letters from the Latin alphabet to represent numbers. Here are the basic Roman numerals and their corresponding values:

I - 1

V - 5

X - 10

L - 50

C - 100

D - 500

M - 1000

To represent numbers, you combine these basic symbols following these rules:

When a smaller numeral appears before a larger numeral, you subtract the smaller from the larger. For example, IV represents 4 (5 - 1) and IX represents 9 (10 - 1).

When a smaller numeral appears after a larger numeral, you add the smaller to the larger. For example, VI represents 6 (5 + 1) and XI represents 11 (10 + 1).

There are limits to repetition: I, X, and C can be repeated up to three times (e.g., III, XXX, CCC), but not more. V, L, and D cannot be repeated.

In no case can a numeral be repeated to the left of another numeral more than ten times its value (e.g., VVV is not valid for 15, you should use XV instead).

Here are some examples of numbers in Roman numeral format:

1 - I

5 - V

10 - X

50 - L

100 - C

500 - D

1000 - M

4 - IV

9 - IX

40 - XL

90 - XC

400 - CD

900 - CM

For larger numbers, you combine these symbols to represent the values. For instance:

50 + 100 = 150, which is written as "CL" in Roman numerals.

500 + 1,000 = 1,500, which is written as "MD" in Roman numerals.

Remember, the biggest numeral system limitation of Roman numerals is their limited ability to represent large numbers clearly, which is one of the reasons they were eventually replaced by more efficient numeral systems like Arabic numerals (the ones we use today).