umlaut
The umlaut is a kind of mutation. Specifically, the vowel vowel, when it comes to the front, is a phenomenon that turns into a related vowel vowel. This phenomenon occurred independently in various germanic languages from 450 to 500 AD and affected all early germanic languages except gothic. Types of umlauts are Ä / ä, Ö / ö, Ü / ü. The international phonetic symbols of each character are ä, ö, ü. If each character is not available, a, o, and u are followed by an e, followed by ae, oe, and ue, respectively.