WTF Fun Fact – Ablaut Reduplication

There is an unwritten English rule concerning ablaut reduplication, or the altering of the interior vowels of a repeated word. When repeating words, the first vowel is always an I, then A or O if there are only two words, and I, A, O if there are three words. For example, it’s “chit-chat” and not “chat-chit”. – WTF Fun Facts

Source: https://www.rd.com/article/ablaut-reduplication/