Siamese twins

As a native English speaker, I take many things for granted that can be a source of puzzlement and frustration for my non-native English-speaking friends and colleagues. 

This week, a conversation I was having with a colleague got me looking into something that I subsequently found out is known in linguistics as an "irreversible binomial", or "Siamese twin." 

These are word pairings that appear in an irreversible fixed order. Examples include "salt and pepper", "ladies and gentleman", and "rock and roll". Flipping the order of these nouns would just be weird. The problem was I couldn't quite explain why we order them they way we do. Something just are the way they are, and I promptly directed my colleague to the wikipedia page on the topic.