Esperanto compounds usually join bare roots directly: skrib-tablo, dent-broso, vapor-ŝipo. Sometimes a word-class ending is retained between the parts to ease pronunciation, reveal a boundary, or preserve meaning: aeroŝipo, postesigno, pagipova, hejmenvojaĝo. These internal endings are often called linking vowels, but they are not meaningless glue; they come from Esperanto's ordinary -o, -e, -i, and sometimes directional -en forms.
The default is economy: omit an internal ending when the compound remains clear and pronounceable. Keep one when established usage favors it or when removing it changes the analysis. All unlabelled forms below are neutral contemporary Esperanto.
Default: join the roots directly
In an ordinary compound, the specifying root precedes the head without retaining its independent ending:
| Separate words | Compound | Not |
|---|---|---|
| skribo + tablo | skribtablo | skribotablo by automatic rule |
| dento + broso | dentobroso/dentbroso | dentojbroso |
| vaporo + ŝipo | vaporŝipo | vaporojŝipo |
La nova skribtablo apenaŭ trapasas la pordon.
The new writing desk barely fits through the door.
La malnova vaporŝipo nun estas mara muzeo.
The old steamship is now a maritime museum.
Plural -j and accusative -n never act as ordinary links. They mark the complete word: dentobrosojn, not dentojbrosojn. Internal endings concern derivational structure, not sentence agreement.
Optional -o- for sound or clarity
After the specifying element, an internal -o- may make a difficult boundary easier to pronounce or understand. PMEG gives pairs such as puŝŝipo/puŝoŝipo, aerŝipo/aeroŝipo, and sangruĝo/sangoruĝo.
Super la haveno malrapide turniĝis granda aeroŝipo.
A large airship slowly turned above the harbor.
La infano desegnis sangoruĝan lunon super la urbo.
The child drew a blood-red moon above the city.
Both the shorter and linked patterns can be structurally valid. Convention decides which one is normal for a particular word. Aerŝipo is compact and clear; aeroŝipo exposes aero + ŝipo. Do not alternate mechanically within one document when an established term has a preferred spelling.
The linking -o- may also reveal a complex first element. Arbaropromenado is easier to segment as arbaro + promenado than arbarpromenado.
La hotelo proponas gvidatan arbaropromenadon ĉiun matenon.
The hotel offers a guided forest walk every morning.
Do not copy a foreign-language final o
An international-looking word may end in -o in several languages, but Esperanto still distinguishes root from grammatical ending. PMEG notes the widespread radioelsendo, influenced by treating radio as indivisible, alongside structurally simpler radielsendo if the intended root is radi-.
La radioelsendo komenciĝos tuj post la novaĵoj.
The radio broadcast will begin immediately after the news.
Established usage can preserve the longer form, so radioelsendo is not an error. The lesson is analytic: do not declare every final -o part of the root or insert it automatically into new compounds.
Borrowed roots and conventional shortenings may have their own structure. Check the actual Esperanto word family rather than the spelling of English, Spanish, or another source language.
No -o- after a quality used directly as modifier
When a quality root specifies the head as a recognized type, the root normally joins directly: dikfingro, altlernejo, sovaĝbesto. The natural expansion uses an adjective—dika fingro, alta lernejo, sovaĝa besto—not a noun in -o.
Mi brulvundis mian dikfingron dum kuirado.
I burned my thumb while cooking.
Ŝi studas biologion en altlernejo.
She studies biology at a higher-education institution.
Forms such as dikofingro and altolernejo impose the wrong internal category. Keeping -a- as dikafingro is not the solution either; use the established direct compound dikfingro, or keep the phrase dika fingro when describing an incidentally thick finger.
Result and manner elements before verbs likewise normally lack a link: ruĝfarbi, plenŝtopi, laŭtlegi. If you want a freely adverbial reading, use two words: legi laŭte.
Rare linking -e- after wordlets
When a short wordlet is the first element, no ending is normally needed: postsigno, antaŭvidi, kunlabori. Rarely, -e- improves pronunciation or prevents a wrong analysis. PMEG contrasts postsigno with postesigno, and postulo “demand/requirement” with posteulo “successor.”
La reĝo nomumis sian filinon posteulo.
The king appointed his daughter as successor.
La kontrakto enhavas neakcepteblan postulon.
The contract contains an unacceptable requirement.
The -e- in posteulo makes the intended poste + ulo analysis visible and distinguishes it from postulo, whose root is postul- “demand/require.” This is lexicalized, not a general instruction to add -e- after every preposition.
Retained -e, -en, or -i can carry meaning
Sometimes an internal ending is semantically essential. Compare antaŭiri, “go before/precede,” with antaŭeniri, “go forward.” The directional -n cannot be discarded without changing the relation.
La prezidanto antaŭiris la delegacion en la ceremonio.
The president went ahead of the delegation in the ceremony.
Malgraŭ la vento la ŝipo antaŭeniris malrapide.
Despite the wind, the ship moved forward slowly.
The same directional package appears in supreniro, ĉieleniro, and hejmenvojaĝo. Shorter supriro, ĉieliro, and hejmvojaĝo also occur where the direction remains recoverable.
La hejmenvojaĝo daŭris pli longe pro la neĝo.
The journey home took longer because of the snow.
With an action followed by pov-, vol-, or dev-, an internal infinitive -i- is common: pagipova, vivivola, pagideva. These are best understood as compressed phrases: able to pay, willing to live, obliged to pay.
La fonduso helpos familiojn, kiuj ne estas pagipovaj.
The fund will help families that are unable to pay.
Forms with -o-, such as pagopova, are possible but less usual in this pattern; forms without any link, such as pagpova, are rare. Here usage, not a universal phonetic rule, determines the normal shape.
Affixes do not take automatic links
When the head is a suffix, or the first element is a true prefix, a linking ending is normally absent: domego, lernanto, malgranda, relegi, not domoego, lernoanto, malagranda, or reo-legi.
Ni relegis la mesaĝon antaŭ ol respondi.
We reread the message before replying.
Affixes already specify a tightly defined relationship to their base. Inserting a word-class ending would create a different or malformed structure. This boundary distinguishes ordinary root compounds from affix stacking.
Linking ending versus word boundary
An internal vowel does not mean two independent words, and the final ending still belongs to the whole compound. Aeroŝipajn biletojn is one adjective describing tickets related to airships; agreement sits on final -ajn.
A hyphen may expose a hard boundary in a long or technical compound, but it does not replace the semantic work of a retained ending. Compare hejmen-vojaĝo, where the hyphen highlights the meaningful directional adverb, with hejmvojaĝo, the compact established alternative. For general hyphen conventions, see word boundaries and hyphens.
Productivity and convention
The system allows variation, but not random variation. Foundationally, internal -o- is optional when it merely facilitates sound or parsing. Accepted usage can conventionalize one form. A retained -e, -i, or -en may be required to preserve a specific phrase relation. Pedagogically, start from familiar dictionary forms and use the shortest unambiguous new compound.
— Ĉu ni skribu aerŝipo aŭ aeroŝipo? — Ambaŭ estas kompreneblaj; ni uzu unu formon konsekvence.
— Should we write aerŝipo or aeroŝipo? — Both are understandable; let's use one form consistently.
In (formal/technical) terminology, consistency across a document matters more than displaying every theoretically possible variant.
Common Mistakes
Preserving plural marking inside a compound
❌ Mi bezonas novan dentojbroson.
Incorrect — internal roots do not retain plural -j.
✅ Mi bezonas novan dentobroson.
I need a new toothbrush.
Adding -o- after every first element
❌ Li vundis sian dikofingron.
Incorrect formation — the quality root joins directly in dikfingro.
✅ Li vundis sian dikfingron.
He injured his thumb.
Dropping a direction that changes the meaning
❌ La ekspedicio antaŭiris tra la dezerto.
Incorrect for 'advanced forward'; antaŭiri means precede or go ahead of something.
✅ La ekspedicio antaŭeniris tra la dezerto.
The expedition advanced through the desert.
Inserting a link before a suffix
❌ La infanoj kuris al la domoego.
Incorrect — -eg- attaches directly to the root.
✅ La infanoj kuris al la domego.
The children ran toward the mansion.
Treating any word-class vowel as a linking vowel
❌ Super la urbo flugis aeraŝipo.
Incorrect compound formation — the accepted alternatives are aerŝipo and aeroŝipo; aera ŝipo would instead be a separate adjective phrase.
✅ Super la urbo flugis aerŝipo.
An airship flew over the city.
Key Takeaways
Direct root joining is the default. Optional -o- can ease pronunciation or parsing, rare -e- can clarify a wordlet boundary, and retained -i- or -en- can preserve the meaning of an underlying phrase. Do not carry internal plural/case endings, do not insert links before affixes, and distinguish a merely helpful vowel from an ending whose removal changes the word.
Related Topics
- Compound Head OrderA2 — Interpret and build ordinary Esperanto compounds with the general head on the right and its modifier before it.
- Transparent Noun CompoundsB1 — Build transparent Esperanto nouns by placing a specifying element before the semantic head, while keeping implicit relations and lexical convention clear.
- Adjective and Adverb CompoundsB1 — Build compound adjectives and adverbs by identifying their semantic head, scope, and phrase-based source instead of copying English word order.
- Verb CompoundsB1 — Build and interpret Esperanto verb compounds by tracking the action head, incorporated modifiers, valency, and conventional meanings.
- Affix Stacking and ScopeB2 — Interpret and build multi-affix Esperanto words by applying each layer in order and keeping the semantic head and participant roles clear.
- Word Boundaries and HyphensA2 — Learn when Esperanto writes elements together, separately, or with an optional hyphen, and how spacing reveals grammatical structure.