I was musing over the difference between a carpet and a rug, and it occurred to me that since Jed has given me a key to this Words & Stuff site, I'll ask y'all what you think.
Personally, I think of a carpet as a semi-permanent fixture, and a rug as being a temporary one. By default, I think of carpet as wall-to-wall, tacked or glued down, and cut in place. By default, I think of a rug as something that can easily be rolled up and carried away. This is true for me, I think, even when the rug is too large to be easily rolled up and carried, but I still think of it as a rug and am likely not to ask for help early enough in the process. But without thinking about it too much, I would have said that in my usage they are different things, and that rarely have to think about which one I would use in any particular case.
This came up in my mind because several times at the library that employs me people have asked to borrow a prayer carpet, and it sounds very wrong to me. The phrase in English is prayer rug, not prayer carpet (Google, for what it's worth, currently shows a 70-to-1 ratio of results for the two phrases, which confirms my sense) and that fits my sense of the difference between a rug and a carpet.
And yet… our prayer rugs, like most of the ones I've seen, are of the style I might call Persian carpet. That phrase is more-or-less as frequent in Google results as Persian rug, and in my personal usage, the terms are more-less-equivalent and call to mind more or less the same image. Similarly with Oriental carpet and Oriental rug, as awkward phrases and as parts of similarly awkward commercial names. And on that front, when I think of a magic carpet or flying carpet, that image is visually much like the prayer rug, and magic rug sounds to me every bit as wrong as prayer carpet.
It's just an idiom, of course, the way English works, with bits of things sounding correct or incorrect, not based on logical rules but on actual usage. The rules (a rug can be laid on a carpet, but not a carpet on a rug) come in to being later, to explain rather than dictate the usage. But what is going on with carpet and rug?
More musings: carpet is a longer, Latin-derived word. Rug is… of uncertain origin. Rug is a kind of textile, then a piece of that textile (a horse rug, f'r'ex), then a mat made out of that stuff, then "a small carpet or mat" (OED). I would have guessed that rug was older, but it isn't. And I do think that carpet is in general a higher-class word (which could account for the fancy imported-from-far-away type of rug being a carpet) except that both a hearth-rug and a lap-rug seem like upper-middle-class rather than working class items, and even the bluest-blooded thoroughbred does not get a horse-carpet.
As for other idioms and combinations that don't refer to textiles at all, I think carpet wins. For one thing, rug isn't a verb (OK, fine, you can rug up a horse, but that's a literal rug) but you can carpet almost anything flat with anything else, including bombs. A person can be called on the carpet (or, in British English at least, carpeted), and there is the delightfully dismissive carpet soldier (or squire or champion or captain) which, for all its derogatory nature, cannot be low class. There are carpet moths and carpet beetles and carpet snakes and carpet sharks, and in the garden there can be carpet beds and further out carpet grass or even carpet weed; I can't think of any rug combinations for flora and fauna at all. It's true that issues can be swept under the rug, but then they can also be swept under the carpet, so that's a wash. The only combinations that come to mind that don't have carpet replacements are rug rats and cutting a rug. Well, and a bad toupee can be called a rug but never a carpet, I suppose.
Before I close, I'll add my surprise to discover (from the OED, of course) that it is rug that is the older slang for pubic hair (particularly women's), with a written entry from 1893 and no such written note for carpet until 1981. I've never heard rug used that way, myself, but maybe I've been reading the wrong things.
So: how do you use those words? Why do you think it became a prayer rug but a flying carpet?