The vowels are divided into strong and weak. They are strong a, o, e ; weak i, u.

It is called diphthong to the fusion of two vowels in a syllable: , , . For there to be a diphthong, it is required that one of the vowels be strong and the other weak not accentuated, or that both be weak.

The combination of strong and weak vowels and vice versa results in fourteen diphthongs in Spanish: , , , , , , , , , , , , , .

Of course, as can be seen from the definition we have given of the diphthong, not always the previous combinations of vowels form it. This happens when the weak vowel is accentuated, as can be seen in the following examples:
, , , , , , , , , , .

It suffices to pronounce the words above correctly, to be convinced that, in them, the combinations of vowels are not part of the same syllable: , . This is because the accent gives the weak vowel a value equal in loudness to that of the strong vowel, and both are, therefore, in the same case as if it were two strong vowels, which can never form a diphthong. , like in , , , , , .

We call it a triphthong to the meeting of three vowels in one syllable. Of those three vowels one must be strong and be in the middle of two weak unstressed.

There are five triphthongs:
, , , , .

When one of the weak vowels appears accentuated, the triphthong does not exist; v. gr.: , .

Carlos González Peña, Manual de Gramática Castellana. Editorial Patria, S.A.(Textual)