Syntagms, paradigms, and a gallon of milk
In semiotics, we sometimes speak of "syntagms" and "paradigms." "Paradigms" are words or signs which are related by the property of being able to substitute or stand in for each other. So, in the sentence
The cat sat on the mat.
"Cat" and "dog" have a paradigmatic relationship with each other, because the word "dog" could substitute for the word "cat." (The sentence wouldn't mean the exactly same thing, of course, but the point is that the new sentence would still be grammatical.)
"Syntagms" are words or signs which are related by the property of appearing together in a group, often in a sequence. So, in the same sentence as above, "cat" and "sat" have a syntagmatic relationship with each other, because they appear next to each other in the sentence.
Clothing is another area where we look for syntagms and paradigms. Two pairs of socks have a paradigmatic relationship, because they can substitute for each other. And socks and shoes have a syntagmatic relationship, since they appear together in the same "group" (i.e., outfit).
So, I actually think about this a lot when I go grocery shopping. (Why, what do you think about when you're grocery shopping?) The reason I think about this, aside from being an incurable nerd, is that grocery stores are largely organized on syntagmatic and paradigmatic principles.
Take cake frosting, for example. Between various brands and flavors, you can probably find 30 different types of cake frosting at a good-sized grocery store. Nobody comes into the grocery store for 30 different types of cake frosting, though — most people just want one or two types. So, why bother to put all 30 together? Why not organize them some other way, maybe alphabetically, so you can go straight to the one you want?
Well, you might come into the store wanting some type of cake frosting, but not knowing exactly what brand or flavor, so the current setup allows you some leeway in that regard. (At least, that scenario seems more likely than coming in wanting either vanilla frosting, vanilla pudding, or vanilla wafers. Not to mention the fact that alphabetizing a list isn't necessarily as straightforward as it seems — do you go by "Vanilla frosting" or "Frosting, Vanilla"?)
Moving down the aisle, why are frosting and cake mix invariably together? Well, one may not be able to substitute for the other, but they do commonly go together. So, even if you don't go to the grocery store looking for either cake mix or frosting, many people do come in looking for both. (This also goes to explain why birthday candles are also in the same aisle.)
Of course, there are limits to the grocery store as a perfect model of syntagms and paradigms. By extension, eggs and milk should also be located next to cake mix, but the practicalities of food preservation intervene and dictate that all refrigerated foods be stored together. (Even if they didn't, some foods are such common staples that they're syntagmatically related to many different other foods. So, while cake mix and frosting are more closely tied with each other than with any other products, eggs and milk are ingredients in many different kinds of mixes and recipes.)
As interesting as it is to ponder the underlying semiotic structure of frosting and cake mix, I find myself really thinking about this sort of thing when I'm looking for something obscure. Take bread crumbs. What are they like? Flour? Corn meal? Bread? (Paradigmatic analysis.) What do they go with? Meat? Vegetables? (Syntagmatic analysis.)
In the end, I think I found them on the same aisle as rice and beans, go figure.