Simplicity and minimalism. The Carl Card Holder is my favourite wallet. It's the one I use daily because it carries a small amount of cash and my three most necessary cards and that's it. It's about 5mm wide when it's full and it slips discreetly into my back pocket. I can't stand having anything wider than this in my pocket; it's uncomfortable and unsightly. Add to these reasons the fact that it's made of genuine leather that keeps looking better and better with age and I think you see why it's so popular.