When zombies attack or when the electric grid is taken down by an EMP or solar flare, trade must go on.

Currently, about 90% of U.S. dollars exist only digitally. Even if fiat cash remained valuable post-apocalypse, it would be pretty hard to come by.

Bitcoin and other cryptocurrencies are great, but still rely on electricity and the internet to function.

So what are the best off the grid currencies for a worst case scenario?