Vowel mutation, also known as ablaut and umlaut, is a process in which the vowel sound of a word is changed to create a new word or to indicate a grammatical change. Ablaut and umlaut are two types of vowel mutation that involve changing the vowel sound of a word. Ablaut involves changing the vowel sound of a word by adding a suffix or prefix, while umlaut involves changing the vowel sound of a word by adding a diacritic mark.