If you get the duvet you will know the measurements. Get the cover just a tad smaller. You don't want the duvet moving around in the cover. If you want to use a comforter you already own then measure it. Buy a cover based on measurements. The cover should be a tad smaller.

If you buy the same size of each it will work. They make clips to secure the comforter to each corner of the duvet so it won't slide. You can find the clips online or at stores such as Bed Bath and Beyond.

If you want a full puffy look use a down comforter (first pic) or a thinner comforter if you like something lightweight that can be folded at the end of the bed.

Many thanks for your most helpful reply. I was at Bed Bath and no one ever mentioned clips!!! You've solved my problem with the perfect answer. I'm most appreciative and hope this may help some others. Have a wonderful day. SF

They sell two different kinds. I recommend this type. Just turn your duvet inside out and clip the corners. They're like mini chip clips.

We tried both kinds of clips sold by BBB and neither held well. We bough a new duvet and comforter a couple years back for a new bed and ordered them from thecompanystore.com, and chose a duvet cover and comforter with ties in the corners, Holds 110% better than the clips. I'm sure you could fashion your own ties pretty easily - I don't really sew but plan to do that for my daughter's duvet cover. Comforters that do not stay in place inside a duvet are a pain in the neck.

I see snaps into mine, not ties, but same concept. On a king size I divide the top edge into thirds and put for snaps, one at each corner and one at 1/3 across and one at 2/3. Keeps the middle part of the duvet from sagging down when you try to pull it up in the night by grabbing the cover.

That said, if you have thin quilts you like, there is no reason you can't do similar to what hotels do these days - instead of a cover make a duvet sandwich with a big flat sheet on the bottom, duvet, big flat sheet or decorative spread on top. Protects almost as well as a cover but less wrestling.

Many thanks for all the comments!!