Roman numerals are a numeral system that was used by ancient Rome. Numbers in this system use letters from the Latin alphabet. Currently, it uses seven symbols:

{| class="wikitable"
|-
! Symbol
| 
| 
| 
| 
| 
| 
| 
|-
! Value
| 1
| 5
| 10
| 50
| 100
| 500
| 1,000
|}

The Europeans still used Roman numerals even after the fall of the Roman Empire. From the 14th century, the Europeans replaced Roman numerals with Arabic numerals. However, people still use Roman numerals to this day.

One place they are sometimes seen is on clock faces (the front of a clock). For example, on the clock of Big Ben, the hours from 1 to 12 are written as:

The  and  can be read as "one less than 5" (4) and "one less than 10" (9). On many clocks that use Roman numerals, however, 4 is written as .

Subtraction rule 
Instead of writing the same letter four times, a rule for subtraction is used. The letter is written once, then the next largest Roman numeral is written. When a lower number (such as ) appears before a higher one (such as ), the lower number is subtracted from the higher one. For example, 4 is not written as , but instead as , because  is  (5) minus  (1). The same is done for 9 - it is not written as , but instead as , because  is  (10) minus  (1).

Example 
It is very easy to write a number as a Roman numeral. Simply subtract the largest possible Roman numeral as many times as possible from the number. This system will result in a valid Roman numeral, but will not take the subtraction rule into account.

Getting the number from the numeral is equally simple, by adding the values of the symbols.

In general, the values for 5, 50, and 500 are not subtracted. Here is the same number using the subtraction rule:

Special values

Zero 
The number zero does not have its own Roman numeral. About 725, Bede or one of his colleagues used the letter N, the abbreviation (short form) of nihil (the Latin word for "nothing").

Fractions 

The Romans also used fractions. The most common base for fractions was 1/12, which in Latin is called uncia (ounce).

Large numbers 
A number of numeral systems are developed for large numbers that cannot be shown with , , , , ,  and .

Apostrophus 
One of the systems is the apostrophus, in which D is written as IƆ (500) and M is written as CIƆ (1,000). In this system, an extra Ɔ means 500, and multiple extra Ɔs are used to mean 5,000, 50,000 etc.
{| class="wikitable centered" style="text-align:center"
|-
! Numeral
| style="width:3em;" | IƆ
| style="width:3em;" | CIƆ
| style="width:3em;" | CIƆƆ
| style="width:3em;" | IƆƆ
| style="width:3em;" | CCIƆƆ| style="width:3em;" | CCIƆƆƆ| style="width:3em;" | CCIƆƆƆƆ| style="width:3em;" | IƆƆƆ| style="width:3em;" | CCCIƆƆƆ| style="width:3em;" | CCCIƆƆƆƆ| style="width:3em;" | CCCIƆƆƆƆƆ| style="width:3em;" | CCCIƆƆƆƆƆƆ|-
! Value
| 500 || 1,000 || 1,500 || 5,000 || 10,000 || 10,500 || 15,000 || 50,000 || 100,000 || 100,500 || 105,000 || 150,000
|}

 Vinculum 
Another system is the vinculum, in which V, X, L, C, D and M are multiplied by 1,000 by adding an overline.

 Usage 

 In the Baltics and Russia, the days of the week are often written as Roman numerals, with I being Monday.
 When writing dates by hand, the month is sometimes written as a Roman numeral, especially for dates written in day-month-year sequence. For example: 26.XI.2014 or XI'.26.2014 = 26 November 2014.
Some games use Roman numerals to indicate the game position in a franchise. The most famous examples are Final Fantasy games (Final Fantasy VII, Final Fantasy VIII, Final Fantasy X, etc...).
 When movies or books are published, the year of publication or year of copyright may be written as a Roman numeral.
 When people write about Monarchs or Popes, Patriarchs, or other leading figures, they are sometimes counted with Roman numbers, e.g. Queen Elizabeth II (of England), Pope John Paul II, Pope Benedict XVI, Patriarch Alexius II (of the Russian-Orthodox church)
 In France, the centuries are sometimes written with Roman numerals (example : "XXe siècle" meaning "20th century", XVIIIe siècle = "18th century", etc...)''. 
 In Poland, roman numerals are used to show the month in dates and as a short method of writing ordinals (i.e.  instead of 6th).
 Unicode has a code block called Number Forms, which also contains representations of Roman numerals, at the positions U+2160 to U+2188.

Related pages 
 Arabic numerals
 Greek numerals

References

Other websites 
 Roman Numerals (Totally Epic Guide) - Know The Romans

Numerals, Roman
Numerals