A Doubler, often also called a booster, is a way of adding more absorbency to a cloth diaper.  A doubler is usually made of 2 to 4 layers of fabric, depending upon the blend and is cut smaller than the main body of the diaper.  The purpose of a doubler is to add absorbency to a diaper for naps, nights, and sometimes for older toddlers. When choosing a doubler you want it to be absorbent enough to work without being so broad and large that it effects the fit at the legs of the diaper, which could ultimately cause leaks.  A doubler or booster that has a stay-dry top on one side of it is also a nice choice if they will be used at nights or for naps.