It's a very basic introductory level text. If you want more detailed answers to questions like that, ask a professor or do a bit more study. Anyway "why don't

It's a very basic introductory level text. If you want more detailed answers to questions like that, ask a professor or do a bit more study. Anyway "why don't we have vector money" is a fairly interesting brain teaser. The way I'd think about that sort of question theoretically is: suppose we tried to actually implement a system like that. Why would people find it annoying / inefficient and go back to the way things were before? Here's how I imagine it going. First, there would have to be an exchange rate between a_dollars and b_dollars, just like for any currency where you want to trade goods priced in one currency for different goods priced in another currency. Then, because it's inconvenient to hold a_dollars and b_dollars, people would just choose to hold onto whichever one was more convenient for them, and make exchanges when necessary. This means whole geographical areas would just use the same currency, because it's just simpler and easier to discard whichever currency is unused. At this point, we enter into the question of optimal currency areas - and basically the whole situation collapses to entire country using a single currency.