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Hive is the base currency of the ecosystem. It can be converted to Hive Power(Influence Tokens) or Hive Dollar. The Hive Dollar is intended to be a stable currency with each Hive Dollar worth 1 US Dollar.

Summarizing a lot:

HIVE: Our naked cryptocoin that is tied to... nothing but the value of our network.

HIVE DOLLAR: Our secondary cryptocoin that happens to have a tendency to stay in pair with US Dollar. It suppose to be emitted more when it's value is above 1 US Dollar and it's not emitted when it's value is below.

One way to look at it is that HBD is $1.00 USD worth of Hive rolled up into it's own cryptocurrency 😁☝️

Thank u for ur answers.