>And the truly great thing about a duvet (provided you're willing to spend the bucks on a really good one) is insulation. Keeps you warm in winter and cool in summer. The cheaper ones work reasonably well in winter, but are lousy in summer. My sister in law used to work at a duvet store here in Toronto, and they had some dillies, about 6 - 8 inches thick that were incredible - winter or summer, but I couldn't justify the cost even with her discount. Of course the other issue is that once one of those is on you, you can't move around much.
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We have ones of different TOGs for summer or winter. I find I get too hot in summer - worst nights you can take out the quilt and just sleep under the cover, like a double sheet on top of you. Duvets are dirt cheap over here. The other thing is you can get matching sheets, duvet covers and curtains, if you're twee enough :-)