English uses the verb “to be” for age, prepositions for dimensions, and “for” before a purchase price. Classical Latin organizes the same information through several different structures: an accusative of extent, a genitive of description, adjectives of dimension, and an ablative or genitive of price. A number is therefore never enough by itself; its noun and case show what is being measured.
Roman units also belong to historical systems. A pēs, lībra, or sēstertius should not be silently treated as an English foot, pound, or modern coin with a fixed purchasing power.
Saying how old someone is
The most transparent Classical age expression uses nātus “born” with an accusative of extent of time:
Lūcia sēdecim annōs nāta est et iam familiam adiuvat.
Lucia is sixteen years old and already helps her household.
Fīlius meus quīnque annōs nātus litterās discit.
My five-year-old son is learning his letters.
Sedecim annōs and quīnque annōs literally measure the span since birth. The participle nātus, nāta agrees with the person, not with annōs. English “is sixteen” omits both “born” and “years,” but Latin normally does not copy that compression.
A genitive of description is also common: puer decem annōrum “a boy of ten years.”
Puer decem annōrum tantum iter sōlus facere nōn dēbet.
A ten-year-old boy ought not make such a journey alone.
This structure packages age as a quality of the noun. With numerals higher than one, annōrum is genitive plural. Another idiom presents the year of life currently in progress:
Mārcus annum vīcēsimum agit; ūndēvīgintī annōs nātus est.
Marcus is in his twentieth year; he is nineteen years old.
Roman counting here is ordinal and inclusive: someone annum vīcēsimum agēns has completed nineteen years and is living through the twentieth. It does not necessarily mean that twenty full years have passed.
Dimensions and extent
Latin commonly puts a measured extent in the accusative, especially with adjectives such as longus “long,” lātus “wide,” altus “high/deep,” and with verbs of distance.
Mūrus decem pedēs altus urbem cingit.
A wall ten feet high surrounds the city.
Fossa quīnque pedēs lāta et trēs pedēs alta erat.
The ditch was five feet wide and three feet deep.
Vīcus duo mīlia passuum ab urbe abest.
The village is two Roman miles from the city.
The measured nouns—pedēs, mīlia passuum—are accusatives of extent. The adjective describing the object agrees with that object: feminine fossa … lāta … alta. It does not agree with masculine plural pedēs. The underlying case construction is treated fully under the accusative of extent of space.
The Roman pēs is a historical foot, divided into twelve unciae. Mīlle passuum is literally “a thousand paces”; plural mīlia behaves as a neuter plural noun and takes genitive passuum. Translating these as “feet” and “miles” is convenient, but exact modern equivalents require a stated standard and scholarly context.
Tabula duōs pedēs longa et pedem lāta est.
The board is two feet long and one foot wide.
The singular measure is pedem, not nominative pēs. English hides this case distinction because “one foot” has the same form wherever it occurs.
Weight and capacity
With goods, the unit phrase takes whatever case the sentence requires. The numeral agrees where it is declinable; the unit noun carries the syntactic role.
Mercātor trēs lībrās oleī et quīnque modiōs frūmentī vēndidit.
The merchant sold three Roman pounds of oil and five modii of grain.
Here librās and modiōs are accusative direct objects. The commodities appear in a partitive or content genitive, oleī and frūmentī. A lībra is a Roman weight; a modius is a dry capacity measure. Neither maps by name alone onto a modern pound or bushel.
In horreō centum modiī frūmentī servantur.
One hundred modii of grain are being kept in the granary.
The values of ancient measures could vary by period, locality, commodity, and administrative standard. That is not a grammatical irregularity. In (formal) legal, military, and accounting texts, the metrological context is part of the evidence.
Prices: definite sums and indefinite value
A definite price is normally ablative:
Hunc librum quīnque dēnāriīs ēmī.
I bought this book for five denarii.
Servus decem mīlibus sēstertium ēmptus est.
The enslaved man was bought for ten thousand sesterces.
The first has ablative plural dēnāriīs. In the second, decem mīlibus is ablative and sēstertium is the contracted genitive plural of sēstertius, dependent on plural mīlibus: literally “for ten thousands of sesterces.” Large monetary expressions often preserve this noun-like syntax of mīlia.
Indefinite expressions of value use the genitive forms magnī “at a high value,” parvī “at little value,” tantī “at so much,” and quantī? “at how much?”
Quantī domum vēndis? Tantī eam vēndō quantī ēmī.
For how much are you selling the house? I am selling it for as much as I paid for it.
Haec tabula magnī aestimātur, sed ego eam parvī aestimō.
This painting is valued highly, but I value it little.
These genitives describe an indefinite valuation, not a counted purse of coins. Once you state a definite sum, use the appropriate money noun in the ablative. The distinction is semantic, not determined by English “for,” which appears in both translations. Compare the separate accounts of the genitive of value and price and the ablative of price.
Coin names and notation hide grammar
Common units include ās (genitive assis), sēstertius, and dēnārius. Their metal content, official relationship, and purchasing power changed over Roman history; “a denarius was worth X euros” is never a purely linguistic equation.
The sign conventionally transcribed HS represents sesterces in Roman accounts and inscriptions. It records a denomination, not a frozen case form. Thus an account’s HS V must be expanded according to its sentence: nominative quīnque sēstertiī as a subject, ablative quīnque sēstertiīs as a purchase price, or another form as syntax requires.
Notae HS V in ratiōnibus scrīptae pretium quīnque sēstertiōrum indicāre possunt.
The signs HS V written in the accounts can indicate a price of five sesterces.
That teaching sentence illustrates expansion, not a claim that every surviving account uses one identical layout. Epigraphic and papyrological conventions vary. Preserve a quoted document’s notation exactly, then explain its grammar separately.
A parsing and production checklist
Before translating a number plus a unit, identify the relation:
| Meaning | Typical Latin structure | Model |
|---|---|---|
| completed age | accusative extent + nātus | vīgintī annōs nātus |
| descriptive age | genitive of description | puer decem annōrum |
| current year of life | accusative year + ordinal + agere | annum vīcēsimum agit |
| dimension or distance | accusative of extent | decem pedēs altus |
| definite price | ablative of price | quīnque dēnāriīs |
| indefinite value | genitive | magnī, quantī |
For production, do not begin by translating English “old,” “by,” or “for.” Decide which relation you mean, choose the Latin construction, and only then decline the numeral and unit.
Common Mistakes
1. Copying English “to be” for age
❌ Mārcus est vīgintī annī.
Incorrect: Classical Latin does not normally copy English ‘Marcus is twenty years.’
✅ Mārcus vīgintī annōs nātus est.
Marcus is twenty years old.
2. Confusing completed age with the year under way
❌ Annum vīcēsimum agit, ergō vīgintī annōs nātus est.
Incorrect: someone in the twentieth year has normally completed nineteen years.
✅ Annum vīcēsimum agit, ergō ūndēvīgintī annōs nātus est.
He is in his twentieth year, so he is nineteen years old.
3. Leaving a singular measure in the nominative
❌ Tabula ūnus pēs lāta est.
Incorrect: measured extent requires accusative pedem.
✅ Tabula ūnum pedem lāta est.
The board is one foot wide.
4. Using the genitive for an exact purchase price
❌ Librum quīnque dēnāriōrum ēmī.
Incorrect for the ordinary statement ‘I bought the book for five denarii’: the exact price is ablative.
✅ Librum quīnque dēnāriīs ēmī.
I bought the book for five denarii.
5. Treating HS V as five in every grammatical context
❌ HS V semper verbīs quīnque sēstertiī redditur.
Incorrect: the notation hides the case required by the sentence.
✅ Pretium HS V verbīs quīnque sēstertiīs exprimitur.
A price of HS V is expressed in words with the ablative quīnque sēstertiīs.
Key Takeaways
Age, dimensions, and prices are relations, not merely numbers. Use accusative extent with annōs nātus and physical measurements, genitive description for “a person of X years,” ablative for exact prices, and genitive for indefinite value. Expand documentary notation only after you know its syntactic role, and treat Roman units as historical measures rather than deceptive modern look-alikes.
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- Ablative of PriceB1 — Express definite prices and things given in exchange with the ablative, while keeping the limited genitive value forms distinct.