The mattress encasement slides over the whole mattress and then is zipped up on one end. If you have a Sleep Number bed, it can't be zipped up all of the way. We zipped it as far as we could and put the sheet on over it and have had no issues with it staying in place/moving around. We actually bought 2. The 2nd mattress was not a Sleep Number mattress and it fit perfectly. Also, even though it is waterproof, there is no feeling of sleeping on plastic or any noise when moving around on it. Overall it's a good mattress cover.