Coinbase maturity

Newly mined coins cannot be spent until the coinbase output is 100 blocks deep (Bitcoin Core's COINBASE_MATURITY, a consensus rule). At average block times that is roughly 16–17 hours. The rule exists because a reorg that orphans a block destroys that block's coinbase entirely — maturity prevents chains of transactions being built on reward coins that might cease to exist.

Common misstatements:

• "Miners can spend their reward immediately." — Consensus forbids spending a coinbase output before 100 confirmations.
• "The 100-block rule applies to all transactions." — It applies only to coinbase outputs; ordinary outputs are spendable in the next block (subject to wallet and relay policy).