I actually knew this. But I couldn't remember the specifics.
There's another thing with letters and why Tick Tock sounds good but Tock Tick sounds terrible.
It was something like, the word with I must come before a word with E or O.
Yeah it's the I-A-O rule. Well, that's what I call it anyway, I don't know what it's called officially. If there are two words, I goes before A or O. If there are three words, the order is I-A-O. Examples of this include Big Bad Wolf (as you can see it defies the adjective rule, because of the I-A-O rule), tic-tac-toe, tick tock, ding dong. It just sounds better to put I before A or O. I think they describe it in this article as well: http://www.bbc.com/culture/story/20160908-the-language-rules-we-know-but-dont-know-we-know
There's another thing with letters and why Tick Tock sounds good but Tock Tick sounds terrible.
It was something like, the word with I must come before a word with E or O.