Short answer: they’re all correct (at least in the United States) but some are more common in certain dialectal areas. Here’s a handy-dandy map, in case you were wondering:
Long answer: I’m going to sort this into reactions I tend to get after answering questions like this one.
What do you mean they’re all correct? Coke/Soda/Pop is clearly wrong. Ok, I’ll admit, there are certain situations when you might need to choose to use one over the other. Say, if you’re writing for a newspaper with a very strict style guide. But otherwise, I’m sticking by my guns here: they’re all correct. How do I know? Because each of them in is current usage, and there is a dialectal group where it is the preferred term. Linguistics (at least the type of linguistics that studies dialectal variation) is all about describing what people actually say and people actually say all three.
But why doesn’t everyone just say the same thing? Wouldn’t that be easier? Easier to understand? Probably, yes. But people use different words for the same thing for the same reasons that they speak different languages. In a very, very simplified way, it kinda works like this:
- You tend to speak like the people that you spend time with. That makes it easier for you to understand each other and lets other people in your social group know that you’re all members of the same group. Like team jerseys.
- Over time, your group will introduce or adopt new linguistic makers that aren’t necessarily used by the whole population. Maybe a person you know refers to sodas as “phosphates” because his grandfather was a sodajerk and that form really catches on among your friends.
- As your group keeps using and adopting new words (or sounds, or grammatical markers or any other facet of language) that are different from other groups their language slowly begins to drift away from the language used by other groups.
- Eventually, in extreme cases, you end up with separate languages. (Like what happened with Latin: different speech communities ended up speaking French, Italian, Spanish, Portuguese, and the other Romance languages rather than the Latin they’d shared under Roman rule.)
This is the process by which languages or dialectal communities tend to diverge. Divergence isn’t the only pressure on speakers, however. Particularly since we can now talk to and listen to people from basically anywhere (Yay internet! Yay TV! Yay radio!) your speech community could look like mine does: split between people from the Pacific Northwest and the South. My personal language use is slowly drifting from mostly Southern to a mix of Southern and Pacific Northwestern. This is called dialect leveling and it’s part of the reason why American dialectal regions tend include hundreds or thousands of miles instead of two or three.
Dialect leveling: Where two or more groups of people start out talking differently and end up talking alike. Schools tend to be a huge factor in this.
So, on the one hand, there is pressure to start all talking alike. On the other hand, however, I still want to sound like I belong with my Southern friends and have them understand me easily (and not be made fun of for sounding strange, let’s be honest) so when I’m talking to them I don’t retain very many markers of the Pacific Northwest. That’s pressure that’s keeping the dialect areas separate and the reason why I still say “soda”, even though I live in a “pop” region.
Huh. That’s pretty cool. Yep. Yep, it sure is.