Summarize this article:

Duvets come in twin, full, queen, and king sizes, so pick a size that corresponds to your mattress size. If you want the duvet to hang over the sides a little more, or you’re really tall, you might consider getting the next size up. The ticking, or the outer part of the duvet, is what holds the filling and is usually made of 100% cotton. It’s important to look at the thread count of the ticking, especially if buying a down or feather duvet. The higher the thread count, the tighter the fabric is woven, meaning it will be much harder for feathers to escape.  Buying a duvet made of cotton ticking, or cotton-polyester, ensures your duvet is super breathable. Aim for a thread count of 200 or more. To keep the filling spread out, sewn through box construction has both sides of the duvet sewn together to create boxes. While this keeps the filling in place, it also doesn't allow filling to reach the edges of each box, making it less warm. In a baffle box structure, small walls are built between each square so that the filling remains spread out while still retaining its warmth. In this construction, there aren't any cold spots. Baffle box construction is more expensive than sewn through box construction due to its more intricate structure. Since duvets aren't easily cleaned, choosing a cover for your duvet will help protect it and make the cleaning process much easier. Duvet covers come in many different materials and styles, allowing you to choose one that matches your bedroom or personal preferences.  Duvet covers are less expensive than regular duvets, making them an affordable option. The covers attach to the duvet with zippers, buttons, or ties.

Summary:
Choose a duvet that will fit your bed size. Pay attention to the ticking and thread count. Decide on sewn through box construction for an affordable duvet. Choose baffle box construction for a more insulated duvet. Protect your duvet by buying a duvet cover.