As it isn't mentioned anywhere that I can see in the product description, I was unpleasantly surprised to find myself unpacking two duvets when my order was delivered; a 4.5 tog 'summer duvet' together with a 9 tog 'winter' one. As I had always avoided these joined duvets in the past, I was concerned about whether they would fasten together properly and stay together (there are four snap fasteners, one in each corner) and whether my rather alarmingly plump conjoined duvet would even fit into my duvet cover! Annoyed, but too lazy to contemplate returning the item, I decided to keep them and give them a try, and am delighted to report excellent results! The current freezing cold snap is really giving this duvet a run for its money, but it's paying off brilliantly and I'm pretty convinced this combo-design is actually warmer than a regular 13.5 tog duvet. The duvets fitted into my standard cover no problem and stayed safely fastened together too and it's great to have the option of separating the duvets for hot summers, should they come, but chilly old bones that I am, I'll probably leave them as they are. I'm only knocking a star off the rating as the two duvets weren't specified, other than that, this is a highly recommended lovely snugly buy.