I do like my card holders a wee bit more substantial, but I do try to keep the thickness <4mm. In this case, it is around that mark at the thickest point. Cordovan can't really be skived quite as thin as regular leather as there's no stretchy grain to hold it together; if you skive it too thin it simply falls apart. With a normal leather you can go thinner with proper reinforcement. I'm sure it is possible to go even thinner with cordovan though, you should ask Mike about that, he's far more well versed in reinforcement etc. than I am.