The poet Horace's full name was Quintus Horatius Flaccus, and his first name is a number: quintus means "fifth". Romans also used Sextus ("sixth") and Decimus ("tenth") as first names, and Decimus Brutus was one of the men who killed Julius Caesar. Whether these names first marked a child's birth order or the month of birth is still debated; either way, by Horace's time they were ordinary first names in common use. But the numbers inside them are still in English, in words you use every week.
One to four
Latin unus, "one", gives uni-: a unicorn is "one-horned", and a unanimous vote is "of one mind" (unus plus animus). For two, Latin has two roots. Duo gives du-, as in dual and duplicate. Bis, "twice", gives bi-, as in bilingual and biennial.
Tres, "three", gives tri-: triangle, triple. Here Greek and Latin happen to agree, since Greek treis also gives tri-, so trilogy is Greek and triangle is Latin. Quattuor, "four", gives quadr-, quadri- and quadru-: a quadrilateral has four sides and a quadruped walks on four feet.
Cardinals and ordinals
Latin has one word for counting ("five", quinque) and another for ordering ("fifth", quintus), and English borrowed from both. Most roots in this deck come from the counting words, but several come from the ordering ones:
| Number | Counting | Ordering | English |
|---|---|---|---|
| 1 | unus | primus | unique, primary |
| 2 | duo | secundus | dual, second |
| 5 | quinque | quintus | quintet |
| 6 | sex | sextus | sextet, sextant |
| 10 | decem | decimus | December, decimal |
Secundus is the odd one out: it comes from sequi, "to follow". The second runner is simply the one who follows.
The months that lost count
September, October, November and December are Latin seven, eight, nine and ten. The early Roman year began in March, so they once sat in those places; January and February now come first, so the names no longer match. Noon has a similar story. It comes from nona hora, "the ninth hour" counted from sunrise, which fell in mid-afternoon. How it slid to midday is still debated.
Hundreds and thousands
Centum, "hundred", gives century, percent ("by the hundred") and centipede. Mille, "thousand", gives millennium and million, which came through Italian milione, "a great thousand". A Roman mile was mille passuum, "a thousand paces", with each pace counted as two steps.
The original metric prefixes, from the 1790s, use both languages in a pattern. Latin prefixes mark fractions: deci- (a tenth), centi- (a hundredth), milli- (a thousandth). Greek prefixes mark multiples: deca- (ten), hecto- (a hundred), kilo- (a thousand). So a millimeter is small and a kilometer is long.
The Greek set
Greek has its own numbers, and English uses them in science and geometry: mono- (one), di- (two), tetra- (four), penta- (five), hexa- (six), octa- (eight), deca- (ten), kilo- (a thousand). A monologue has one speaker, carbon dioxide has two oxygen atoms, and a pentagon has five angles. Some words mix the two: bicycle is Latin bi- with Greek kyklos, "circle".
Common mistakes
- Reading every nov- as nine. November counts; novel, novice and renovate come from novus, "new".
- Taking centipede and millipede literally. Centipedes have an odd number of pairs of legs, so never exactly 100, and most millipedes have far fewer than 1,000 legs.
- Mixing up biennial and biannual. Biennial is every two years. Biannual usually means twice a year, though usage varies, so check which one a writer meant.
- Assuming decimate once meant "wipe out". Decimare meant picking out one man in ten for punishment. The wider sense grew up in English from the 1600s.
Frequently asked questions
What are the Latin number prefixes from 1 to 10?
uni- (1), du- or bi- (2), tri- (3), quadr- (4), quint- (5), sex- (6), sept- (7), oct- (8), nov- (9) and dec- (10). Several also come in an ordinal form, like quint- from quintus, "fifth".
Is mono- Latin or Greek?
Greek, from monos, "alone". The Latin equivalent is uni-, from unus.
Why is October the tenth month if oct- means eight?
The early Roman year began in March, which made October the eighth month. When January and February were counted first, the name stayed and the number moved.
Why does the metric system use both Latin and Greek prefixes?
The original prefixes of the 1790s follow a pattern: Latin for fractions (deci-, centi-, milli-) and Greek for multiples (deca-, hecto-, kilo-), so the prefix tells you which way you're going. Later prefixes break it: micro-, a millionth, is Greek.
Try it
Say each number before you flip the card. Then go back to Horace: what number was hidden in his first name, and which English word for a group of musicians still carries it?