Roman numerals use seven letters of the Latin alphabet and each of these letters has a numerical value.
These are the Roman numerals and their numerical correspondences:
I = 1
V = 5
X = 10
L = 50
C = 100
D = 500
M = 1.000
What do we use Roman numerals for today?
Today, we use this numbering system used in ancient Rome in the following contexts:
1.- The numbering of the centuries. The Roman numeral should follow the noun “siglo”.
Let’s see some examples: “siglo XV” (“siglo quince”); “siglo XX” (“siglo veinte”); “siglo I a.C.” (“siglo uno antes de Cristo” / “siglo primero antes de Cristo”).
2.- To number and to differentiate kings and popes that have the same name. The Roman numeral will be written after the name of the monarch or the pope.
Let’s see some examples: “Felipe VI” (“Felipe sexto”); “Carlos V” (“Carlos quinto”); “papa Juan Pablo II” (“papa Juan Pablo segundo”); “papa Juan XXIII” (“papa Juan veintitrés”).
3.- To number the dynasties of certain cultures or civilizations. In this case, the Roman numerals will go before the noun “dinastía”.
Example: “los faraones de la IV dinastía” (“los faraones de la cuarta dinastía”).
4.- To number the edition of a congress, fair, festival, contests, etc.
Examples: “XX Festival de Jazz” (“vigésimo Festival de Jazz”); “X Feria de la Ciencia” (“décima Feria de la Ciencia”)
5.- To number the parts of a work or publication (volumes, books, chapters…)
Examples: “libro III” (“libro tercero”); “capítulo V” (“capítulo quinto”).
6.- To write the year on a commemorative monument.
Example: MCMLXX = 1970
Some considerations about its writing and reading
1.- They are always written in capital letters, except if they are used in the numbering of a section or a list. Only in this case, it could also be written in lowercase.
2.- Depending on its placement, the value of that number will be added or subtracted. We add when the value of the next number is equal to or less than the one that precedes it and we subtract otherwise.
Examples: IV (4); VI (6); IX (9); XI (11)
3.- Currently, it isn’t allowed that the same Roman numeral is written more than three times in a row.
We don’t write: “IIII”, “XXXX” nor “CCCC”.
4.- It isn’t allowed to write the same Roman numeral twice in a row if there is already a numberal with that value. In other words, we don’t write “LL” (100) but “C “(100).
5.- Roman numerals can be read as an ordinal number but also as a cardinal number, depending on the context.
6.- The numerals accompanying the names of kings and popes are read as an ordinal number up to the Roman numeral X, thereafter they are read as a cardinal number.
We say: “Felipe V” (“Felipe quinto”), “Alfonso XII” (“Alfonso doce”), “papa Juan Pablo II” (“papa Juan Pablo segundo”), “papa Juan XXIII” (“papa Juan veintitrés”).
7.- Both the numerals referring to a year and to the centuries from I to X can be read as an ordinal number or as a cardinal number, but from XI onwards they will be read as a cardinal number.
We say: “año V” (“año quinto” or “año cinco”); “año M” (“año mil”); “siglo II” (“siglo dos” or “siglo segundo”); “siglo XII” (“siglo doce”).
Bibliography consulted: RAE, Ortografía básica de la lengua española; Leonardo Gómez Torrego, Ortografía de uso del español actual