One of the convenient features of banknotes is their fungibility. This means that one banknote is perfectly interchangeable with another. For a few months now, something strange has happened to Yemen's banknotes. Old rials and new rials have ceased to be fungible. Any rial note that was printed prior to 2016 is now worth around 10% more than newer rial notes.

More generally, the entire Yemeni monetary system has split on the basis of banknote age. From a Western perspective, it would be as if every single U.S. banknote issued with a Steve Mnuchin signature on it, the current Treasury Secretary, were worth 10% less than bills signed five years ago by his predecessor Jack Lew.

Conflicts are always complicated. What follows is a short but drastically simplified explanation of how Yemen's banknote problem began