Strong Vowels and Weak Vowels

Some combinations form diphthongs and triphthongs


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Vowels in Spanish are classified as either weak or strong, and the classification determines when combinations of two or more vowels are considered to form a separate syllable.

Key Takeaways: Spanish Vowels

  • The strong vowels of Spanish are a, e, and o; the weak vowels are i and u.
  • When two strong vowels are next to each other, they form separate syllables; in other combinations, the vowels are in the same syllable.
  • Two vowels next to each other form a diphthong; three vowels next to each other form a triphthong.

Two Types of Vowels

The strong vowels of Spanish — sometimes known as open vowels—are a, e, and o. The weak vowels — sometimes known as closed vowels or semivowels—are i and u. Y often serves as a weak vowel as well, functioning in the same way and sounding the same as i.

The basic rule of vowel combinations and syllables is that two strong vowels cannot be in the same syllable, so that when two strong vowels are next to each other, they are considered to belong to separate syllables. But other combinations — such as a strong and a weak vowel or two weak vowels — form a single syllable.

Be aware that in real life, especially in rapid speech, two strong vowels, such as in the words maestro and Oaxaca, often slide together to be pronounced in a way that may sound like a single syllable or very close to it. But they are still considered separate syllables for writing purposes, such as when dividing words at the end of a line or for the use of accent marks.

Keep in mind that vowel sounds in Spanish tend to be purer than they are in English. In English, for example, the word "boa" (a type of snake) often sounds something like "boh-wah," while in Spanish boa sounds more like "boh-ah." This is because English speakers often pronounce the long "o" with a slight "ooh" sound at the end, while Spanish speakers do not.


When a strong and a weak vowel or two weak vowels combine to form a single syllable, they form a diphthong. An example of a diphthong is the ai combination in baile (dance). The ai combination here sounds much like the English word "eye." Another example is the ui combination in fui, which to the English speaker sounds much like "fwee."

Here are some fairly common words that include diphthongs (shown in boldface): puerto (port), tierra (earth), siete (seven), hay (there is or there are), cuida (care), ciudad (city), labio (lip), hacia (toward), paisano (peasant), cancn (song), Europa (Europe), aire (air).

In some words, a strong and weak vowel or two weak vowels don't merge together but instead form separate syllables. In those cases, a written accent over the weak vowel is used to show the distinction. A common example is the name María. Without the accent mark, the name would be pronounced much like MAHR-yah. In effect, the accent mark turns the i into a strong vowel. Other words where an accent mark is used to keep a weak vowel from becoming part of a diphthong include río (river), heroína (heroine), dúo (duet) and país (country).

If there is an accent over the strong vowel, it doesn't destroy the diphthong. For example, in adiós, the accent merely indicates where the spoken stress goes but doesn't affect how the vowels work together.


Occasionally, a diphthong can combine with a third vowel to form a triphthong. Triphthongs never have two strong vowels in them; they are formed by either three weak vowels or a strong vowel with two weak vowels. Words that have triphthongs include Uruguay (Uruguay), estudiáis (you study) and buey (ox).

Note that for purposes of the written accent, the y is considered a consonant even if it is functioning as a vowel. Thus the final syllable of Uruguay is what gets the stress; that's where the stress goes on words ending in a consonant other than n or s. If the final letter were an i, the word would need to be spelled Uruguái to maintain the pronunciation.

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Erichsen, Gerald. "Strong Vowels and Weak Vowels." ThoughtCo, Aug. 27, 2020, Erichsen, Gerald. (2020, August 27). Strong Vowels and Weak Vowels. Retrieved from Erichsen, Gerald. "Strong Vowels and Weak Vowels." ThoughtCo. (accessed February 5, 2023).