3 Diphthongs and triphthongs
As in English, two vocalic segments in Spanish can form a single syllable. It sounds as if the vowels were pronounced rapidly, cuatro, ['ku̯atro] (four) with one of the elements actually losing its full vowel status. In Spanish, even three vowels can form a single syllable; these are called triphthongs, e.g. cambiáis [kam.ˈbi̯aːi̯s] (you-pl. change). Most words in Spanish containing a triphthong are verbs in second person plural vosotros (this form only exists in the Peninsular Spanish variety).
Rules of syllabification
A diphthong (or triphthong) is formed when an unstressed high vowel is adjacent to another vowel (generally a mid or low vowel). In these cases, the high vowel loses its syllabic status and a so-called ‘glide’ is formed. Sequences which rise in aperture (glide + vowel) are called rising diphthongs, while vowel + glide sequences are called falling diphthongs.
The rising diphthongs of Spanish are /u̯a, u̯e, u̯o, i̯a, i̯e, i̯o/, as in pascua (Easter), muela (tooth), cuota (share), historia (history), piel (skin), limpio (clean).
The falling diphthongs of Spanish are /ai̯, ei,̯ oi̯, au̯, eu̯, (ou̯)/, paisaje (landscape),rey (king), boina (beret), flauta (flute), Europa (Europe). Note that when the glide is word-final, it is generally spelt ‘y’. Some transcriptions use /j/ rather than /i̯/.
A homogenous diphthong is created when two different high vowels come together as in viudo (widower) and cuidado (care). They are generally pronounced as rising diphthongs.