DiphthongsHiatuses
From Ial
[edit] Diphthongs and hiatuses
A diphthong is a combination of two different vowel sounds that can be pronounced one after the other, without interruption between the first and the second. In other word the articulation of the first vowel moves seamlessly to the other.
Conversely, a hiatus is a group of two near vowels that separate.
If you only have three vowels a, i, u, you can glide from a to i or u, which gives you two diphthongs: ai and au. But note i-a, i-u, u-a and u-i are all hiatuses, that is these vowel combinations can only be articulated separatedly and thus they belong to separate syllables. In our language this remains true even if i,u are preceded by a consonant, e.g. ti-a (this is not a one-sylllable /tja/ sound, there is not j or y sound and even if there were we would have written tja, not tia).
No more than six two-vowel combinations are possible with three vowels.
For instance,Spanish has diphthongs, hiatus and syllable division( strong vowels: a, e, o and weak vowels: i,u). Two strong vowels beside are called hiatus(e.g. in the word correo the last vowels "eo" create a hiatus). A strong vowel and a weak vowel next to each other are named a diphthong (e.g. puerto, "ue" build a diphthong).