There are two types of vowels, i u or "weak vowels"(unstressed) and a e o or "strong vowels". A diphthong is the combination of a strong vowel and a weak vowel (in either order) or of two weak vowels together.

1. Vowel combinations involving a e o are always divided:

ca-no-a

le-o

tra-e

cre-e

2. When unstressed, i and u unite with each other, or a e o to form DIPHTHONGS. (The two vowels are pronounced as a single sound, with one of the vowels being given slightly more emphasis than the other. In all diphthongs the strong vowel or the second of two weak vowels receives this slightly greater stress):

pien-sa

bue-nos

ciu-dad

gra-cias

3. If i or u has a written accent it does not combine with other vowels to form diphthongs and consequently forms a separate syllable:

dí-a

te-o-rí-a

tí-o

ba-úl

4. If unstressed i or u is the first or third component of three contiguous vowels, a TRIPHTHONG results:

a-ve-ri-guáis

en-viéis