This pack includes four of our most poplular sized plastic coil 6mm, 8mm, 10mm, and 12mm. We Include 10 pieces of each for a total of 40 pieces that can make 40 books. Each size works with a different size of book. This is perfect for DIY or small business use where a whole pack of 100 is not needed. Also great when you only need a couple of books and are not sure what size will fit best.