Catalan uses the diaeresis only on i and u: ï, ü. It has two jobs. It can show that a high vowel forms a separate syllable rather than a diphthong (veïna, llaüt), or it can show that u is pronounced after g or q before e or i (aigües, qüestió).
The two jobs share one logic: the dots prevent the reader from applying the default interpretation of the surrounding letters. They do not mark ordinary stress, and they are not interchangeable with acute or grave accents.
For the underlying syllable contrast, see Diphthongs, Hiatus, and the Diaeresis.
A quick sorting table
| Pattern | Correct form | Instruction |
|---|---|---|
| vowel + syllabic i/u | raïm, veïna, llaüt | keep the vowels in separate syllables |
| g/q + u + e/i | següent, pingüí, qüestió | pronounce the u |
| stress already needs an accent | país, veí, Lluís | the acute accent takes priority |
| systematic exception | agrair, egoisme, reiniciar | pronounce hiatus without writing a diaeresis |
La meva veïna ha comprat raïm i un llaüt antic.
My neighbor has bought grapes and an old lute.
La següent qüestió la respondrà una lingüista.
The next question will be answered by a linguist.
Job one: marking hiatus
When i or u follows a vowel and must remain a syllable nucleus, a diaeresis normally marks the hiatus: ra-ïm, ve-ï-na, lla-üt, a-mo-ï-nar. It may also separate the high vowel from both neighbors, as in con-du-ï-a and a-gra-ï-en.
La notícia amoïnava els veïns del barri.
The news was worrying the neighborhood residents.
Quan conduïa de nit, reduïa la velocitat.
When she drove at night, she reduced her speed.
This spelling can distinguish words. Beneit “foolish” has a diphthong, while beneït “blessed” has hiatus. Reina means “queen,” whereas the uncommon noun reïna means “resin.” The mark is therefore capable of preserving both syllable structure and lexical identity.
El pa beneït era damunt la taula.
The blessed bread was on the table.
When an acute accent takes priority
If the high vowel must carry a written accent under the general stress rules, use í or ú, not a diaeresis. Compare país with països, veí with veïna, and traíem with traïa. Both marks can imply hiatus, but only the accent also records required stress.
| Accented form | Diaeresis relative | Family |
|---|---|---|
| país | països | country / countries |
| veí | veïna, veïnat | neighbor / neighborhood |
| traíem | traïa | we betrayed / he or she betrayed |
| suís | suïssa | Swiss (masculine / feminine) |
En Lluís ha viscut en tres països diferents.
Lluís has lived in three different countries.
Job two: making u audible
In gue, gui, que, and qui, written u is normally silent: guerra, guitarra, que, qui. A diaeresis changes the instruction to “pronounce this u”: güe, güi, qüe, qüi. The result is normally [w] within the same syllable, not a separate u syllable.
El pingüí neda en aigües molt fredes.
The penguin swims in very cold waters.
És una qüestió freqüent entre els estudiants bilingües.
It is a frequent question among bilingual students.
Spanish also uses gü in güe/güi, but Catalan additionally needs qüe/qüi: qüestió, aqüífer, terraqüi. Writing Spanish-like cuestió or omitting the dots changes the Catalan spelling system.
The major morphological exceptions
The hardest part is that some predictable hiatuses deliberately remain unmarked. These are not random omissions; they preserve visible morphemes and avoid covering frequent paradigms with dots.
Third-conjugation verb endings
Do not write a diaeresis in the infinitive, gerund, future, or conditional endings of third-conjugation verbs when a vowel precedes them: agrair, agraint, agrairé, agrairia; conduir, conduint, conduirem, conduiries. The hiatus is still pronounced.
Other forms do take it when the relevant i is no longer inside those exempt endings: agraït, agraïa, agraïssis; conduïa, conduït.
T'ho voldria agrair abans de marxar.
I'd like to thank you before leaving.
Mentre conduïa, anava agraint l'ajuda a tothom.
While she was driving, she kept thanking everyone for the help.
Prefixes, compounds, and certain suffixes
No diaeresis is normally written at a transparent boundary between a prefix and its base, or between compound elements: reiniciar, antiincendis, contraindicat, tallaungles. The same exemption applies in true -isme and -ista derivatives such as egoisme, egoista, altruisme, and altruista.
Han reiniciat el sistema antiincendis.
They have restarted the fire-protection system.
This is where morphology defeats sound alone. Altruisme has no dots because -isme is an identifiable suffix, but proïsme and lluïsme take them because their endings are not that productive suffix in the same analysis. Current IEC practice also omits the diaeresis in learned derivatives in -al such as coital and romboidal. Forms such as aïllar, aïrar, and reüll are lexicalized exceptions and keep the mark.
Latin-type endings supply another fixed exception: aquàrium, mèdium, pòdium, Màrius have hiatus without a diaeresis.
Editing exercise
Correct the message La linguista del meu pais conduia cap a la universitat i volia agraïr l'ajuda a tothom. Decide separately whether each high vowel needs an accent, a diaeresis, or an exception with no mark.
❌ La linguista del meu pais conduia cap a la universitat i volia agraïr l'ajuda a tothom.
Incorrect — the sentence confuses accents, diaereses, and an exempt infinitive.
✅ La lingüista del meu país conduïa cap a la universitat i volia agrair l'ajuda a tothom.
The linguist from my country was driving to the university and wanted to thank everyone for the help.
The apostrophe in l'ajuda has no spaces, and a word such as col·labora would need a true middle dot rather than a period. Those signs cannot substitute for the diaeresis.
Common Mistakes
1. Omitting hiatus dots.
❌ La veina compra raim cada dissabte.
Incorrect — veïna and raïm require a diaeresis.
✅ La veïna compra raïm cada dissabte.
The neighbor buys grapes every Saturday.
2. Using a diaeresis where an accent takes priority.
❌ Aquest païs té un aqüífer enorme.
Incorrect — país requires acute í.
✅ Aquest país té un aqüífer enorme.
This country has a huge aquifer.
3. Copying Spanish spelling after q.
❌ Aquesta cuestió és ambigua.
Incorrect — Catalan writes qüestió with an audible ü.
✅ Aquesta qüestió és ambigua.
This question is ambiguous.
4. Adding dots to an exempt verb form.
❌ T'ho vull agraïr personalment.
Incorrect — the infinitive agrair belongs to the systematic verb-ending exception.
✅ T'ho vull agrair personalment.
I want to thank you personally for it.
Key Takeaways
Use ï/ü to mark a normally unexpected pronunciation: hiatus or audible u in güe/güi/qüe/qüi. If stress rules require í/ú, the accent takes priority. Learn the systematic exceptions by structure—verb endings, prefix and compound boundaries, -isme/-ista, certain learned -al derivatives, and Latin endings—rather than treating them as an arbitrary list.
Related Topics
- Diphthongs, Hiatus, and the DiaeresisA1 — Learn when adjacent Catalan vowels share a syllable, when they separate, and what the diaeresis tells you to pronounce.
- I and U as Vowels and GlidesA2 — Recognize when Catalan i and u form a syllable nucleus and when they become the glides [j] and [w].
- When Catalan Words Take a Written AccentA1 — Decide when Catalan stress must be marked by classifying words as aguda, plana, or esdrúixola.
- Acute and Grave AccentsA1 — Choose the correct Catalan accent direction and use é, è, ó, and ò to preserve stressed vowel quality.
- Accent and Diaeresis ErrorsA2 — Correct missing and misdirected accents, hiatus and pronounced-u diaereses, and diacritics that change Catalan word meaning.