• Open (strong) vowels: a, e, o
• Closed (weak) vowels: i, u

•Diphthong (diptongo): 2 vowels pronounced in the same syllable. There are two types:

	info	examples
open vowel + closed vowel or vice versa	The closed vowel cannot be stressed, otherwise there is no diphthong	ciencia, cambio, puente, guapa, izquierda, nació
closed vowel + closed vowel	Both vowels must be different	viuda, triunfo, altruista

•Triphthong (triptongo): 3 vowels pronounced in the same syllable.

	info	examples
closed vowel + open vowel + closed vowel	The closed vowels cannot be stressed: e.g. abríais (there is not a trphthong)	despreciáis, santiguáis

•Hiatus (hiato): 2 vowels together which are not part of the same syllable.

	info	examples
open vowel + open vowel		paella, dehesa
closed vowel + closed vowel	Both vowels being the same	chiita
open vowel + closed vowel	provided that the closed vowel is stressed	María, día leíste

The hiatus bears accent as long as it is allowed by the general rules of accentuation except when in a open vowel + closed vowel structure or vice versa, if the closed vowel is stressed, it bears an accent on it even if the rules do not allow it; e.g. María, dúo (llanas although ending in vowel); Raúl, reír (agudas although ending in consonant other than -n or -s)

•Compound words

• If it has two or more terms, it behaves as one word: tiovivo, ciempiés
• If the compound words are joint by a hyphen, both words keep their accents: crítico-biográfico, físico-químico
• Adverbs ending in -mente keep their accent if they had it before adverbialization: ágilmente (ágil), tímidamente (tímida), accidentalmente (accidental)