Cotton casings are by far the most popular form of casing for a modern day silk duvet. Cotton casings are available in various grades but it is important to consider thread count when making a purchase. Generally the higher the thread count, the finer the stitch and the better wearing the duvet will be. Anything over 240 thread count should be fine. Anything under and one should question the quality and continued durability. Quality of casing is often directly related to price.