Fitted nappies are the most absorbent nappy system available in cloth nappies because they are made completely of absorbent materials which is then covered by a wrap to make it waterproof. Fitted nappies can easily be boosted with more pieces of absorbent materials so can last a really long time before needing to be changed, so are perfect for nighttime or long car journeys.  LittleLamb fitted nappies are designed to fit perfectly inside LittleLamb wraps. We designed our wraps not only to fit our nappies perfectly but also with other brands of real nappies in mind so we believe it’s probably the best all-rounder. The wrap is the waterproof aspect of the fitted system. We take baby-soft polyester and bond a urethane film to the inside - PUL. The film, under a microscope, would look a bit like a layer of baked bean tins on your countertop. They are pressed together but as they are round there are still gaps between. The gaps are big enough to let air molecules pass through but small enough to stop all the larger fluid molecules. We suggest purchasing one wrap for every three nappies as you only need to change the wrap when it is soiled. We do not recommend using the same wrap for more than 12 hours and we believe you should always use a fresh wrap for nighttime. Wraps are only needed for our bamboo and cotton fitted nappies. You don't need a wrap for the onesize or pocket nappies.