The official suffix -on- forms fractions. The numeral before -on- is the denominator: it states into how many equal parts a whole is divided. Duono is one of two equal parts, triono one of three, and centono one of one hundred.
This order can feel backwards to English speakers because English often names a fraction with an ordinal-looking word such as third. In Esperanto, tri-on-o transparently means “one part resulting from division by three.” The suffix belongs only with numerals and number-like expressions. All unlabelled uses below are neutral contemporary Esperanto.
The denominator is inside the derived word
The ending determines how the fractional meaning functions:
| Form | Word class | Meaning |
|---|---|---|
| kvarono | noun | one quarter; one of four equal parts |
| kvarona | adjective | equal to or constituting one quarter |
| kvarone | adverb | to the extent of one quarter |
Mi manĝis duonon de la sandviĉo kaj konservis la reston.
I ate half of the sandwich and saved the rest.
Nur triono de la volontuloj povas veni sabate.
Only one third of the volunteers can come on Saturday.
La rezervujo estas kvarone plena.
The tank is one quarter full.
The fraction noun is the semantic head. The whole commonly follows with de, particularly when it is definite: duono de la kuko, triono de la buĝeto, kvarono de la loĝantoj.
The base number is not the thing being divided
Triono does not mean a part of the number three. It means one of three equal parts of some other whole. This prevents forms such as landono for “part of a country.” A country is not a denominator; use landparto.
La norda landparto ricevis la plej multe da pluvo.
The northern part of the country received the most rain.
Triono de la lando restis sen elektro.
One third of the country remained without electricity.
The first identifies a region without claiming equal division. The second makes an exact or approximate fractional claim: the affected area or population represents one of three equal shares. Parto is general; -on- mathematically structures a whole.
The same distinction separates -on- from -er-. An ero is one repeated constituent unit; an ono is an equal share defined by division. A grain of sand is sablero, not sablono.
Numerators greater than one stay outside
The derived noun itself denotes one fractional part. Put a cardinal before it to state a larger numerator, and pluralize the fractional noun:
Ni jam finis du trionojn de la projekto.
We have already completed two thirds of the project.
Tri kvaronoj de la ĉeestantoj subtenis la proponon.
Three quarters of those present supported the proposal.
La aparato perdis kvin centonojn de sia kapacito dum la provo.
The device lost five hundredths of its capacity during the test.
In du trionojn, du is the numerator and tri inside trion- is the denominator. The plural -j marks multiple fractional parts, and the accusative -n marks the object of finis. Cardinals themselves do not take -j or -n.
The theoretically regular unuono equals one whole divided by one, but it is extremely rare because plain unu expresses the result. Productivity does not make every mathematically possible form communicatively useful.
Adjective forms and fractional measures
An -on-a form directly describes a noun as one fractional unit:
Ni faru duonhoran paŭzon antaŭ la posttagmeza prelego.
Let's take a half-hour break before the afternoon lecture.
La saŭco bezonas kvaronan litron da buljono.
The sauce needs a quarter of a liter of stock.
Ŝi legis nur trionan parton de la kontrakto.
She read only a one-third portion of the contract.
The adjective agrees normally. Kvaronan litron has accusative marking on both words because the measured quantity is the direct object. In duonhora, the whole phrase duona horo has been compacted before the adjectival ending: a break whose duration is half an hour.
Mixed quantities can use a fractional adjective:
La kunveno daŭris du kaj duonan horojn.
The meeting lasted two and a half hours.
This expands conceptually to du horojn kaj duonan horon. The fractional adjective is singular in meaning because it modifies one understood horo, while the final measured expression is conventionally plural and bears the accusative of duration.
Adverb forms: fractional extent
An -on-e form says to what fraction an action, state, or composition holds:
Mi nur duone komprenis la klarigon.
I only half understood the explanation.
La glaso estas duone plena, sed la botelo estas preskaŭ malplena.
The glass is half full, but the bottle is almost empty.
For parallel composition statements, speakers also say trione el ligno, trione el vitro, trione el metalo. When the numerator exceeds one, a noun phrase such as du trionojn can make the exact proportion clearer. In everyday (informal) use, duone may be figurative rather than measured: duone ŝerce means “half jokingly.”
— Ĉu vi serioze proponas tion? — Nur duone; mi ankoraŭ pripensas la detalojn.
— Are you seriously proposing that? — Only partly; I am still considering the details.
Context distinguishes exact half from looser partial extent.
Fractional versus ordinal meaning
An ordinal places something in a sequence; -on- divides a whole. Dua parto is the second part, while duona parto is a part whose size is one half.
Mi spektis la duan epizodon, sed nur duonon de ĝi.
I watched the second episode, but only half of it.
This minimal contrast matters because English a second can name either an ordinal item or a time unit, and many European fraction names resemble ordinals. Esperanto keeps du-a “second” and du-on-a “half” morphologically distinct.
Prefix-like use and lexicalized duon-
Fraction words can stand before another root: duonhoro is half an hour, kvaronkilometro a quarter kilometer, and trionparto a one-third part.
La stacidomo estas nur kvaronkilometron for de ĉi tie.
The station is only a quarter kilometer from here.
Some duon- compounds have conventional extended meanings. Duonfrato is a half-brother—someone sharing one parent—not half of a person. Duonfermita means half-closed or partly closed. These remain motivated by partialness, but their exact interpretation is lexical.
Mia duonfrato kaj mi havas la saman patrinon.
My half-brother and I have the same mother.
Do not assume every noun accepts a natural duon- compound. Duono de la raporto is clearer than a novel duonraporto if you mean half of a particular report; the latter could suggest an incomplete or hybrid kind of report.
Complex denominators and spelling
The complete denominator joins -on-. Simple forms are written together: dekduono “one twelfth,” sesdekono “one sixtieth.” Hyphens can preserve boundaries in a long numeral: tricent-sesdek-kvinono “one three-hundred-and-sixty-fifth,” mil-ducentono “one twelve-hundredth.”
Unu centimetro estas centono de metro.
One centimeter is one hundredth of a meter.
La libro havas 365 paĝojn, do unu paĝo estas tricent-sesdek-kvinono de la tuto.
The book has 365 pages, so one page is one three-hundred-and-sixty-fifth of the whole.
A space changes the structure. Dek-duono with a clarifying hyphen is one twelfth; dek duonoj is ten halves. The first is one derived denominator, while the second is a cardinal plus a plural noun.
-On-, -obl-, and -op-
The three number suffixes correspond to three different structures:
La kvaropa familio mendis duoblan porcion sed manĝis nur tri kvaronojn de ĝi.
The four-member family ordered a double portion but ate only three quarters of it.
Kvaropa gives group size; duobla multiplies a reference portion; kvaronoj divides that portion into four equal shares. The number before each suffix is interpreted according to the suffix, not according to an English translation.
For a full treatment of numerator agreement, mixed measurements, and clock expressions, see fractions.
Common Mistakes
Using an ordinal for a fraction
❌ Mi uzis duan de la faruno.
Incorrect — dua means second in a sequence, not one half.
✅ Mi uzis duonon de la faruno.
I used half of the flour.
Forgetting plural fractional parts
❌ Kvin sesono de la tasko jam estas finita.
Incorrect — five sixth-parts require the plural sesonoj.
✅ Kvin sesonoj de la tasko jam estas finitaj.
Five sixths of the task is already complete.
Treating -on- as a general word for “part”
❌ La norda landono havas montojn.
Incorrect — the region is not an equal share defined by a numerical denominator.
✅ La norda landparto havas montojn.
The northern part of the country has mountains.
Reversing numerator and denominator
❌ Du kvaronoj signifas kvar duonojn.
Incorrect arithmetic and analysis — du kvaronoj equals one half.
✅ Du kvaronoj egalas unu duonon.
Two quarters equal one half.
Separating a derived denominator
❌ Unu dek duono de la prezo estas kvin eŭroj.
Incorrect spacing for the denominator twelve.
✅ Unu dek-duono de la prezo estas kvin eŭroj.
One twelfth of the price is five euros.
Key Takeaways
-On- means division: the number before it is the denominator, and the result is one equal share of another whole. Put a larger numerator outside and pluralize the fractional noun: du trionoj. Use -ona for fractional measures and -one for fractional extent, preserve the complete denominator inside one written word, and choose parto rather than -on- when no equal numerical division is intended.
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Start learning Esperanto →Related Topics
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- -op-: Groups Of A Stated NumberB1 — Use -op- to turn a number into a group size, distinguishing members acting together from simple counting, distribution, repetition, and multiplication.
- FractionsA2 — Build exact fractions with the official suffix -on-, then use their noun, adjective, and adverb forms with quantities and units.
- Cardinal NumbersA1 — Use Esperanto cardinal numbers to count, quantify noun phrases, identify items, and read basic dates, times, measurements, and decimal notation.
- Numbers as Nouns, Adjectives, and AdverbsB2 — Change Esperanto number words into nouns, adjectives, and adverbs while preserving the distinction between quantity, order, and manner.
- Count and Mass MeaningsA1 — How Esperanto distinguishes countable individuals from unbounded substances and shifts nouns between those meanings.