b. One of two contiguous vowels accented.

(1) _When two contiguous vowel's are strong._

(a) There is no diphthong if one of two contiguous strong vowels
receives the accent.

Ex. _Chispë|ando el sol hiere_, p. 173, XXVI I, l. 17.

Ex. _Tú, sombra a|érea que, cuantas veces_, p. 170, XV, l. 7.

By synaeresis, however, a diphthong may be formed, especially in the
combinations á^o, á^e, ó^e--_c^a^o^s, c^a^e, ro^e_. But in order to
diphthongize oa, ea, and eo, when the accent naturally falls on the
first vowel, the accent must shift to the second, which is a dominant
vowel. Such diphthongization is harsh. For example, _loa_ would shift
the accent from o to a in order to form a diphthong. The accent would
also shift in _cre^a, fe^o_.

(2) _When one of the contiguous vowels is weak and the other strong._

(a) There is no diphthong if an accented weak vowel precedes a strong.

Ex. _Yo, que á tus ojos en mi agoní|a_, p. 171, XV, l. 18.
Synaeresis is, however, sometimes employed to overcome this rule.
The accent must then shift.

Ex. _Habi^a llegado una nave._ Calderón.

(b) There is no diphthong if an accented weak vowel follows a strong.