The question is if that's small enough. With a maximum of 21 MBTC, the smallest unit corresponds to 1/5e16 of that. As a comparison, there's roughly 10 trillion USD in existence, where the smallest (physical) denominator is 0.01 USD, corresponding to 1e-15 of the total. Thus, bitcoin's "advantage" here is minimal.

It is small enough for now, in my opinion.

When we get to the point in time where a μBTC becomes common denomination for buying goods (like mBTC is now) then maybe it will be time to start thinking about nBTC and pBTC. But we are long ways away from that time, so no need to fiddle with it now.

In my opinion.

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And the smallest denomination of USD being a penny is ridiculous. Our inflationary currency has made them worthless, we really should get rid of them. Hell, I'd even be for getting rid of the nickel too but we can't because of the quarter and dime.