Imagine if there was not a strong central Government with a Federalized US Mint -- instead, there was a single emperor or ruler who had authority over a few states, but not others -- and every state was allowed to design and mint their own coins, in whatever metal content they desired, and create their own denomination systems. Then also allow really large cities and even churches and bishoprics to also design and mint coins, sometimes in open defiance of the Emperor. The end result in the Holy Roman Empire was over 380 entities (countries, churches, duchies, cities) issuing coinage in a 300-year span.
Eventually, enterprising merchants and nobles brought about "customs unions" that helped create baseline denominations for trading between states, and the single-ruler Government also created a baseline currency for receipt of taxes and revenues from its client states.
Example: If Florida was a member of the "Empire" and Alabama was not, your Alabama coins would only be worth their metal content in Florida, and vice-versa, unless they were joined together in a "customs union" that agreed on a common monetary standard. However, if Georgia was also a member of the "Empire", trading between Florida and Georgia would not be a problem.
As the single ruler and his Empire lost power and prestige due to military setbacks and shifting political alignments, the best-armed, most militaristic and most industrialized of the states (i.e. Prussia) filled the growing leadership void, and formed alliances with -- or outright conquered -- other states and European nations, and took away the rights of the bishoprics to mint their own coins. The power of the State's laws and decrees to set the monetary standard grew and grew until eventually they controlled every state, unifying them under a single currency, with no need for customs unions.
That's pretty much the German States collecting environment from the late 1500s through the late 1800s, and the reasons why it's possible to stay shallow (a few coins here and there) or go very, very deep.
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