A diaper cover is a water-proof or water resistant cover that is worn over a fitted, prefold, or flat diaper. A cover can also be the outer shell used in an All in Two or Hybrid system. A diaper cover typically has elastic at the legs and back of the waist, and closes at the waist or hips with either snaps or a hook and loop fastener. A cover made of PUL or TPU fabric will be waterproof. Covers made from nylon, fleece and wool are also available and are water resistant. Covers are available in both sized and one-size options. A good-fitting cover can be the backbone of a cloth diaper stash. One-size covers tend to be a bit bulky at the smaller adjustments, but the flexibility of adjusting up (and sometimes down) in size as baby grows can be a very beneficial (and cost-saving) feature.