I'm sure someone else can explain this more eloquently, but I believe the basic idea is this: any time you make a transaction that isn't settled on the spot, at

I'm sure someone else can explain this more eloquently, but I believe the basic idea is this: any time you make a transaction that isn't settled on the spot, at least one of the parties is extending credit. like suppose one person has more grain than they need. everyone else is starving, but they have nothing to trade for grain. if the grain person agrees to let the other villagers have grain now in exchange for the promise of an ox next year, they have in a sense expanded the money supply. this type of arrangement can be implicit and difficult to track. maybe there was no explicit promise of an ox, but an unspoken understanding that a favor would be owed in the future. it still has the effect of expanding the money supply without digging rare metals out of the ground.