👉 Diphthongation is a grammatical rule in English, which means that two or more vowels (the "diphthongs") are placed at the beginning of a word. This is similar to how two consonants (the "consonants") are placed at the start of a word in English. For example, "happy" and "happily" both have three diphthongs: /k/ , /θ/, and /d/.
