### Roman Numerals

The user can convert numbers in Roman numeration or hexadecimal (the main menu item File|Convert Number). The use of Roman numerals has been mathematically obsolete for more than 1100 years. Nonetheless, the Roman symbols for numbers continue to be used in a variety of ways, most of them rather stereotyped: to mark the hours on clock faces, to number pages in the prefaces of books, to express copyright dates.

The basic Roman numerals are:

 I = 1 V = 5 X = 10 L = 50 C = 100 D = 500 M = 1000

The symbols are repeated to form larger numbers (not more tha 4000), and when different symbols are combined, the larger unit precedes the smaller. Some numerals can repeat (I, X, C, M), but not more than three times. For example, the number 283:

`CCLXXXIII,`

i.e. 200+50+30+3=283. Here the symbol representing 100 was repeated twice, the symbols representing 10 and 1 were repeated by three times.

The Romans usually wrote IIII for 4 and XXXX for 40. To shorten the length of numbers a "subtraction rule" appeared in later Roman times and was commonly used in medieval times. The "subtraction rule" allows the use of six compound symbols in which a smaller unit precedes the larger:

 IV = 4 IX = 9 XL = 40 XC = 90 CD = 400 CM = 900

Other "subtracted" symbols are not allowed. Thus 99 must written XCIX, not IC.

Using Roman numeration the large numbers (more than 4000) can be written. A bar over a number multiplies that number by 1000, a double bar multiplies that number by 1000000. For example, the number 123123:

```______
CXXIIICXXIII
```