Two vowels coming together constitute a _diphthong_; three, a
_triphthong_. In Irish there are _thirteen_ diphthongs and _five_
triphthongs. Of the former, _six_ are always long, or naturally so, and
the remainder naturally short, but becoming long when accented. The long
diphthongs, as they are invariably long, require no accent. The _seven_
short need the presence of this character to show that their sound is, in
the case so designated, to be pronounced long.
The long diphthongs are:—_ae_, _ao_, _eo_, _eu_, _ia_, and _ua_.