Spanish vowels are divided into two classes: the strong vowels, _a_,
_o_, _e_, and the weak vowels, _u_, _i_. According to the Academy rules,
followed by most grammarians, there can be no diphthongization of two
strong vowels in the proper pronunciation of prose; only when a strong
unites with a weak or two weaks unite can diphthongization take place.
In verse, on the other hand, diphthongization of two strong vowels is
not only allowable but common. This would probably not be the case if
the same thing did not have considerable justification in colloquial
practice. As a matter of fact we frequently hear _ahora_ pronounced
_áora_ with diphthongization and shift of stress.
Of the three strong vowels, _a_ is "dominant" over _o_ and _e_; _o_ is
dominant over _e_; and any one of the three is dominant over _u_ or _i_.
A dominant vowel is one which has the power of attracting to itself the
stress which, except for diphthongization, would fall on the other
vowel with which it unites. The vowel losing the stress is called the
"absorbed" vowel. This principle, which we find exemplified in the
earliest poetic monuments of the language, must be thoroughly understood
by the student of modern Spanish verse.