Mark Forsyth on BBC:
You are utterly familiar with the rule of ablaut reduplication. You’ve been using it all your life. It’s just that you’ve never heard of it. But if somebody said the words zag-zig, or ‘cross-criss you would know, deep down in your loins, that they were breaking a sacred rule of language. You just wouldn’t know which one.
All four of a horse’s feet make exactly the same sound. But we always, always say clip-clop, never clop-clip. Every second your watch (or the grandfather clock in the hall makes the same sound) but we say tick-tock, never tock-tick. You will never eat a Kat Kit bar. The bells in Frère Jaques will forever chime ‘ding dang dong’.
Reduplication in linguistics is when you repeat a word, sometimes with an altered consonant (lovey-dovey, fuddy-duddy, nitty-gritty), and sometimes with an altered vowel: bish-bash-bosh, ding-dang-dong. If there are three words then the order has to go I, A, O. If there are two words then the first is I and the second is either A or O. Mish-mash, chit-chat, dilly-dally, shilly-shally, tip top, hip-hop, flip-flop, tic tac, sing song, ding dong, King Kong, ping pong.