Latin Numbers Grammar: Which Decline, Milia, and 18 and 19

Latin is famous for its endings, and its numbers are where it quietly lets you off. Most of them never change. The whole grammar of Latin counting is three small words that decline, a subtraction at eighteen and nineteen, and one noun, milia, that wants a genitive after it.

One to ten: three that decline, the rest that never do

There is no zero. Classical Latin had no number word for nothing; a Roman said nullus (no, not any) or nihil (nothing): Nihil habeo. - I have nothing.

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1. 1unus
2. 2duo
3. 3tres
4. 4quattuor
5. 5quinque
6. 6sex
7. 7septem
8. 8octo
9. 9novem
10. 10decem

Count the apples from unus to decem. Only the first three change to agree with what they count.

The first three agree with what they count in case and gender. From four on, the number is indeclinable, and the noun carries the case on its own:

• duae partes - two parts (pars is feminine)
• quattuor partes - four parts
• cum quattuor partibus - with four parts

Unus: one, only, and a genitive of its own

Unus declines like bonus except in two places, which it shares with nullus, solus and totus: the genitive singular is unius and the dative uni, for all three genders.

• unus deus - one god
• una pars - one part
• unum nomen - one name

Unus often means a little more than one: a single one, or the only one, leaning towards solus (alone). Unus ex multis is one out of many, singled out. It even has a plural for nouns that exist only in the plural: castra, a camp, is plural in form, so one camp is una castra.

Duo and tres

Duo keeps traces of an old dual number, which is why it looks like nothing else:

The masculine accusative is sometimes duo as well, but duos is the form to learn. Tres is a regular third-declension plural, with one form for masculine and feminine and one for neuter:

• duae partes - two parts
• tria nomina - three names
• trium dierum - of three days

Get these two tables right and you have done nearly all the declining Latin numbers ask of you below two hundred.

The teens, and why eighteen counts backwards

Eleven to seventeen are a unit fused onto ten, with some wear at the join:

Then Latin changes direction. Eighteen is two-from-twenty and nineteen is one-from-twenty, and the de in the middle is the preposition from:

The pattern runs at the top of every ten:

Decem et octo, ten and eight, does turn up for eighteen (the Vulgate uses it), but the subtractive forms are the classical norm.

The tens and a hundred

All indeclinable. Watch the vowel before -ginta: septuaginta has a u, octoginta an o, and nonaginta starts like nonus (ninth) rather than novem. English kept one of them: the Greek Old Testament is the Septuagint, after the seventy translators of the legend (the story actually has seventy-two, rounded down).

Joining them: 21 to 99

Latin gives you two ways to build a compound under a hundred, and both are correct:

• viginti unus - twenty-one, bigger number first, no et
• unus et viginti - one and twenty, smaller first, joined by et

Above a hundred the bigger number leads: centum viginti tres, 123. If the compound contains unus, duo or tres, that part still agrees with the noun, and the noun is plural:

• viginti tres dies - twenty-three days
• viginti duae partes - twenty-two parts

Hundreds and thousands

From two hundred to nine hundred the hundreds are plural adjectives, declined like the plural of bonus:

So three hundred agrees with its noun: trecenti with men, trecentae with parts, trecenta with names.

A thousand is where Latin does something English does not. Mille is an indeclinable adjective; its plural milia is a neuter noun, and what it counts goes into the genitive:

The number in front of milia agrees with it as a neuter plural, which is why it is tria milia, never tres milia. The Roman mile is the commonest place you will see mille itself with a genitive. There is no classical word for a million: a Roman multiplied, decies centena milia, ten times a hundred thousand.

For first, second and third, see Latin ordinal numbers.

Words are not numerals

The number words are the language; Roman numerals are a way of writing quantities with seven symbols, I, V, X, L, C, D and M. They are as different as the word seven and the digit 7.

They do not follow the same logic either. Subtracting a smaller symbol before a larger one (IV, IX) is found in Roman inscriptions, but plain addition (IIII, VIIII) is common too; IV as the one correct four is a later tidying-up. And the words subtract in different places: undeviginti is one-from-twenty, while XIX is ten plus one-from-ten.

If you meet a numeral in a Latin text, read it aloud as the word, in the right form for its noun: III partes is tres partes.

Mistakes English speakers make

What to do next

1. Say each number aloud with a noun attached: tres dies, duae partes, tria nomina.
2. Run the drill below until the forms of duo and tres come without thinking.
3. Read every Roman numeral you meet aloud as the word.