What is a fitted diaper you ask? Think: All-In-One but it needs a cover and that will give you the basic idea. A fitted diaper is usually cotton on the outside and on the inside and the middle has something absorbent sewn into the middle such as zorb. A cover is needed because the cotton is not waterproof in any way. I have seen these diapers made in a sized version (large, medium, small, etc) and in a One Size version (snaps on the front to adjust the rise). I have also seen these made with snap closure, Velcro closure, and no closure sewn on, in which case you could use a Snappi or a diaper pin.Why get a fitted instead of an AIO? In my opinion, a fitted diaper is the best diaper for night-time use. About 2 months ago, I discovered the joy of these diapers. An AIO diaper is only absorbent on that small strip under the baby where the insert is sewn in. A fitted diaper is completely absorbent. True, when you take it off the entire thing will be soaked (if you use it overnight), but at least you don’t have any leaks! I love these diapers at night-time. I do add a fleece liner to the inside of mine to wick moisture away from Baby “S”, but I love having a diaper that is completely absorbent and a great cover to keep it from leaking.