* **Origin of 21M**: **Satoshi Nakamoto** chose the **21 million Bitcoin** cap by setting an initial block reward of **50 BTC** halving every **210,000 blocks** (approx.  4 years), resulting in a geometric series summing to 21 million.
* **Early Code**: Some analyses suggest early source code (v0.0) had a limit of **1.99 billion Bitcoin**, but this was changed to 21 million before the January 2009 launch.