Spanish Diphthongs and Hiatus: When Two Vowels Make One Syllable

Spanish words often contain two or three vowels in a row, and whether those vowels share a syllable or not affects pronunciation, rhythm and spelling. The rule behind it is simple and depends on just one idea: some vowels are strong and some are weak.

Strong and weak vowels

Spanish divides its five vowels into two groups:

• Strong (open): a, e, o
• Weak (closed): i, u (and y at the end of a word, as in hoy)

When an unstressed weak vowel stands next to another vowel, it turns into a glide, [j] or [w], and the two vowels are pronounced in one syllable. That is a diphthong. When two strong vowels meet, each keeps its own syllable. That is a hiatus.

The diphthongs

The key to sounding natural is to keep the glide short. bien is one syllable, [bjen], not "bee-en". bueno is [ˈbwe.no], two syllables, not three. English speakers tend to split diphthongs when they read slowly, which makes speech sound laboured.

Hiatus: two syllables

A hiatus happens in two situations.

1. Two strong vowels

teatro is te-a-tro, poeta is po-e-ta, leer is le-er, caos is ca-os. In relaxed speech many speakers pull these together, especially in Latin America (teatro can sound close to [ˈtja.tɾo]), but careful speech keeps them apart.

2. A stressed weak vowel

If the i or u is the stressed vowel, it cannot become a glide, so it forms its own syllable. Spanish always marks this with an accent, even when the normal accent rules say otherwise:

Other common hiatus words: día, país, río, baúl, maíz, oído, frío, tía. Many are verb forms in -ía: tenía, vivía, sabría.

Triphthongs

A weak + strong + weak sequence forms a triphthong, all in one syllable: buey, Uruguay, Paraguay, averiguáis, estudiáis. The last two are vosotros forms, used in Spain.

Silent h does not separate vowels

Because h is silent, it does not prevent a diphthong: prohibir is pronounced [pɾoj.ˈβiɾ] and ahumado [aw.ˈma.ðo]. When the weak vowel is stressed, the accent is still written, even across the h: prohíbe, búho, rehúsa.

Meanings: envió "he sent" / envío "shipment"; sabia "wise (woman)" / sabía "knew"; media "half" / medía "measured"; hacia "towards" / hacía "was doing".

Frequently asked questions

What are the strong and weak vowels in Spanish?

The strong (open) vowels are a, e and o. The weak (closed) vowels are i and u. A weak vowel next to any other vowel normally forms a diphthong; two strong vowels together are always in separate syllables.

Is "ui" in muy a diphthong?

Yes. Two weak vowels together form a diphthong: muy, ruido, ciudad, cuidado. The stress usually falls on the second one.

Why does "guion" have no accent?

Since the 2010 spelling rules, words like guion, truhan and fie count as one syllable for accent purposes, because i + o is treated as a diphthong. One-syllable words take no accent, so the older spelling guión is no longer standard, though some speakers pronounce two syllables.

Sources and further reading

• Real Academia Española and ASALE (2010). Ortografía de la lengua española, sections 1.2 and 3.4 on diphthongs, triphthongs and hiatus. Espasa.
• Hualde, J. I. (2005). The Sounds of Spanish, chapter 5: Glides and diphthongs. Cambridge University Press.
• Hualde, J. I. and Prieto, M. (2002). On the diphthong/hiatus contrast in Spanish: some experimental results. Linguistics, 40(2), 217–234.

Read More in This Topic

• Spanish Stress and Accent Mark Rules: Agudas, Llanas and Esdrújulas
• Spanish Vowels: How to Pronounce the Five Pure Vowels
• Spanish Linking and Sinalefa: Why Native Speakers Sound So Fast

Disclosure: this page links to Preply, a tutoring marketplace. If you book a lesson through that link we may earn a commission, at no extra cost to you. It does not change what is recommended here. See our disclaimer and privacy policy.