While stocking a new apt at BedBath&Beyond (a rare excursion), I saw a huge sign saying “5 reasons to buy a duvet cover”.
None of the five reason explained whether I HAVE a duvet or why I might NEED a duvet.
And if I have one, why should I cover it? Is it something I should be ashamed of? 
Check out my thread from last week…
http://boards.straightdope.com/sdmb/showthread.php?threadid=112903
So NOBODY knows for sure? 
The knower of all things house;
Martha Stewart
You should have seen what vB code did to that link before I disabled smilies.
“smile”
Peace,
mangeorge
Awww, what the hell, yojim -
I’ll bite. We recently purchased a beautiful huge warm toasty (white) down comforter. We have 2 cats. We moseyed over and bought a cover (a nice olive green) to put OVER the beast. It is soft and comfy, and every couple of weeks we can TAKE THE COVER OFF and throw it in the washer. Good as new. Now, try throwing your down comforter in the washer. Your wife will kill you, and it will never dry. It’s also quite expensive to clean down bedspreads. Basically, the answer is: “Why do you wear shoes?..” Because they protect my feet from nastiness and I don’t want to run to the podiatrist every fortnight to fix something that wouldn’t have happened if I was wearing my Converse.
Also, you can change the color scheme of your boudoir and just get another cover.
My next thread: “What is a ‘pillowcase?’”
I like my duvet. It’s both lighter and warmer than a blanket - for the same reason down-filled coats are lighter and warmer than woolen coats.
My duvet cover, I admit with shame, does not see a lot of use. But it’s used for the same reason you put a bed-cover on a regular bed - just so the bed, when made, will look nice and so that you can wash the part of the bed-clothes that shows without having to clean all the feathers, etc.
(Tyler Durden)
You know what a duvet is? It’s a blanket, just a blanket.
(Tyler Durden)
It is not a duvet. It covers a duvet.
Cretins! 
Peace,
mangeorge (Want’s Martha Stewart)
Thank you! A cover for a blanket (whatever it’s made of) is sufficiently clear.
My theory is that making the bed in the morning (or ever, actually) will merly attarct dust to the uniform surface, probably through some kind of electrostatic action. Leaving a rat’s nest of covers that is stirred nightly will prevent the charge and dust buildup. So I guess I don’t need a duvet cover,
That’s my story and I’m stickin’ to it.
It’s a pillow case for your comforter.
No, its a duvet cover for a duvet. Pillow cases cover pillows.
What’s a duvet? What’s a comforter? We call them Doonas. (Don’t ask me why, 'cause I don’t know).
Don’t know why you call them doonas, we (New Zealanders) call them duvets (doo-vay).
Because you steal words from other languages and then can’t spell them?
It’s a dyne, as it was in the beginning, is now and ever shall be, world without end, so there.
Doesn’t anybody here have a dictionary? Oops, they’re online:
Ame. Her. Dic.: du·vet n. A quilt, usually with a washable cover, that may be used in place of a bedspread and top sheet. [French, down, from Old French, alteration of dumet, diminutive of dum, dun, from Old Norse d¿nn.]
The cover in intergal to the duvet.
You would NEVER use the duvet without one.
And the cover is removed (at LEAST weekly) to be washed.
The duvet you only wash on very special occasions, if you have a washing machine which can take it.
This is the only name I’ve ever heard used for bed coverings, and it is definitely NOT the same as a comforter, which is just a blanket which just sits there to look pretty. The comforter is rarely used to sllep under.
Well, I’m going to make one (duvet cover), using two sheets, just like Martha showed me. Velcro and all. But I’m putting a couple of old blankets inside.
Then I’m going to invite her over to try it out. Think she’ll know the difference? You know, like “The Princess and the Pea”.
I’ll fix her eggs benedict for breakfast. And Mimosa.
Anybody want to know how things work out?
Peace,
mangeorge
does no-one call them quilts anymore?
A quilt is not the same thing as a duvet. With a quilt you usually have a top sheet , two or three blankets and then the quilt on top. With a duvet that is all you have, no top sheet and no blankets. Hence the need for a duvet cover that can be washed on a regular basis just like you would with a top sheet. Most duvet cover designs come as a matching set with pillow cases and sometimes curtains (drapes). You can get them ( if you want )with cartoon characters on or your favourite football team design.