The Castilian grammar contains a multitude of possibilities, as in the case of examples of diphthongs. In fact, we understand by diphthong that word formed from the unit of two vowels.
Thus, for example, it is possible to find this same association between open or closed vowels, taking into account that open vowels are understood to be the vowels a, e, o, while in the case of closed vowels they would be i, u respectively. In addition, there are two possibilities regarding the typology of this concept, that is, those that are decreasing and others that are increasing.
Examples of diphthongs we can find in words like:
citizen, feudal or careful.