So that you can easily stick a foot out the edge for temperature adjustment. Especially with heavy winter duvets, the traditional overhang can be oppressive.
(Related: In Europe it’s common to actually have a personal duvet (twin size) for each of the bed occupants. I’ve seen it referred to as the “Scandinavian sleep method.”)
This post explains the two duvet option, which I also think is legit:
See also Advice: innovation for couples sharing a bed
But go ahead and measure: Ikea duvet covers have a standard 86” length across all three sizes—that means that if you’re someone who likes to pull the covers over your head and/or tuck your feet like the bottom of a sleeping bag, you may need greater width in order to be able to run your setup at a diagonal, even if a twin size would otherwise work for you personally or for your full-size bed.
Here's a nice synthetic duvet (350 gsm fill)
(maybe better for allergies? — I've heard mixed opinions here because on the one hand, bird feathers seem like they would be bad for allergies, but on the other hand, a good down comforter should have a very tight weave so that none of the feather bits can escape, and that might act roughly like a dust mite cover — where there isn't a natural reason for a synthetic duvet to need such a tightly woven cover)
And here's one that's slightly thinner (250 gsm fill) and so maybe better for summer and/or warmer climates?
Note: Duvet covers are weirdly expensive. I don't really understand why — shouldn't it be about twice the cost of a sheet? I really love the feel and durability of the percale options at LLBean and I like that they have ties in the corners to connect with the loops on the duvet, but I can't resist the diversity of prints (and the lower price tag) of the options at Ikea even though they're typically not as soft. (and they'll ship for $6 (or for free if you hit the $50 minimum for small items), so you don't even need a nearby store)
