The letter X is used to show you which combs have what thickness
choices. It can be seen as a lighter or heavier comb you can choose and
is relative to that model. Some people know that they like a certain
weight or solidity to a tool they use, if you don't have a preference
you can ask for help as to what thickness Doctorlonghair who had these
carved, recommends. Other than that if you like a rich feeling n a
comber ...go up in x'x ...a scritcher?...stay 1x or slender.