The plain or twill weave of flannel sheets also makes them soft and pliant — all the more reason to use them in colder weather.  

Make sure you understand the distinction between a duvet and a duvet cover. It might not be easy to distinguish between the two primary components of a duvet set while shopping without prior knowledge. Keep in mind that the duvet is the fluffy blanket contained within the set, and the duvet cover is the warm shell surrounding the duvet. For a complete duvet set, you'll need both.