Question: why a e i o u are called vowels?
Answer: Out of 26 letters in English language five letters, i.e., a, e, i, o, and u are considered as vowels because, these letters independently or combining together perform the function of vowel sound in the spelling of a word. For example, 'cat' is a word; here the sound of vowel /æ/ is denoted by the letter 'a'.