Circulating Supply shows how many satoshis have been issued by the Bitcoin protocol up to the current date. It is the canonical supply series in this family and the native-unit monetary base from which issuance, inflation, and market-cap metrics are derived.
A rising series means total issued supply is still increasing, though the slope slows after each halving. It is most useful with New Supply Issued, Annualized Inflation Rate, and Stock-to-Flow, where the question is stock versus current flow.
The key caveat is conceptual, not technical. This is protocol-defined issued supply, not an estimate of economically active coins. It is not adjusted for lost, dormant, or inaccessible BTC.
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