Coinbase Transaction

Every block starts with a single coinbase transaction — the one transaction allowed to create bitcoin out of thin air. Its single "input" doesn't reference any previous output; instead, it carries an arbitrary 2–100 byte field (the "coinbase scriptSig") where miners typically write a message, a pool tag, or extra nonce bytes for hashing.

The outputs pay the miner two things:

1. The block subsidy — a fixed amount of newly issued bitcoin set by the protocol. Began at 50 BTC, [halves](/glossary/halving) every 210,000 blocks. Currently 3.125 BTC after the 2024 halving.
2. The transaction fees from every other transaction in the block. Each one contributes its (input - output) difference.

The coinbase output is subject to a 100-block maturity lock. The miner can't spend it until 100 more blocks have been built on top, to prevent disruption if a deep reorg eventually orphans the block.

The most famous coinbase is the very first one, in the genesis block. Its scriptSig contains the encoded headline:

> *The Times 03/Jan/2009 Chancellor on brink of second bailout for banks*

A timestamp, and an editorial. By convention, the genesis coinbase output (50 BTC to the address 1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa) is unspendable — Satoshi's reference implementation bug meant the output's database entry was never added to the UTXO set, even though anyone with Satoshi's keys could in principle spend it.