A flat is a large single ply piece of fabric which can be folded to form a diaper.  Most often made of cotton, flats can also be found with bamboo or hemp in the fabric’s blend.  A flat will need to be secured with pins or Snappi and will need a diaper cover to be waterproof. Many trim-fitting modern cloth diaper covers can be paired with a pad-folded flat to make a trim and functional diapering option that is adjustable and washes and dries easily.  Flats are available in a variety of dimensions depending upon the manufacturer.  This is most relevant when the flat is going to be folded and fastened on the baby. If flats will be folded into a rectangular shape and laid in the diaper cover, then size is less of an issue, though larger flats do tend to be bulkier when folded in this way.