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.
Ex. ¿Cómo puede re|ir? p. 182, XLIX, l. 4.