The design of these hand carders has evolved over the years, culminating in a great piece of equipment that won't break the bank.
The backs are made of birch ply, allowing for a small amount of flex, reducing stress on the woodwork. To help with this the card cloth is not glued down to the base, instead it is stapled to the wood. This means that the cloth can pull and relax with the carding action. The card cloth itself uses stainless steel wire so you can be sure that they won't rust, and your carders should last for many years to come.
In general Standard Carders are recommended for most wools. These have the card cloth set at 72 Points Per Inch (PPI): any more and the friction between the 2 carders can make them hard to use. Fine Carders can be used for finer wools and silks with the cloth at 108 PPI.