Tu glorïosa frente.
(6) Two contiguous weak vowels with the accent on the first of them form an indissoluble diphthong; e.g., xxv muy. Cuita, cuido and related forms once accented the u: cf. [p. 134], l. 20 where Cervantes has descuido in assonance with confuso. So also, Meléndez Valdés assonated tumba and cuidan. Viùda was formerly víuda, and Tirso de Molina assonated it with Lucía, pican, etc.
(7) If the second of two contiguous weak vowels is accented, there is a diphthong sometimes indissoluble and sometimes dissoluble; e.g., indissoluble are fuí and, in modern usage, cuita, cuido and their derivatives; dissoluble are ruin, ruina, ruido, viudo. These later, however, readily admit synæresis.
Analogy operates in verb forms; thus u is in a syllable apart in huyo, arguyo, and so also in huimos, argüimos (but in such cases synæresis is always possible). In cases of a repetition of the same vowel, synæresis hardly obtains; therefore piísimo and duúnviro have four syllables each.
II. Combinations of Two Vowels with the Accent Preceding them
(1) Two contiguous strong vowels after the accent naturally form two syllables: e.g., Dánao, héroe, temiéndoos. Yet the poets usually make diphthongs of them; e.g., Moratín:
Los héroes que la fama
Coronó de laureles,
and only exceptionally treat them as dissyllabic; e.g., Samaniego:
Cuando á un héròè quieras