Twenty-three days: joining numbers, and words versus numerals

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

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.