## What is the coinbase?

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I searched the bitcoin wiki and found references to coinbase, but no clear cut explanation of what exactly it is. I know that miners can tag their coinbases (that is how some sites tell who mined a block).

1For questions about the company see the tag [tag:coinbase.com]. – Murch – 2015-11-07T15:20:15.127

4Not to be confused with the Bitcoin startup Coinbase, which is a hosted (shared) EWallet + exchange. – Stephen Gornick – 2012-11-26T05:12:14.550

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The term "coinbase" is used to mean many different things. But the two you're probably asking about are:

1. The "coinbase transaction" is the transaction inside a block that pays the miner his block reward.

2. Inside the coinbase transaction is a field that is called the "coinbase". It's the generation transaction's equivalent of a scriptsig. Since it doesn't claim any existing outputs, it needs no normal scriptsig. It's basically just a random value that the miner can use as an additional nonce. BIP 34 changes this a bit.

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There's also now a Bitcoin exchange/hosted wallet web site called http://www.coinbase.com.

– gosmond – 2013-02-21T04:25:17.220

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David has already given a good explanation of the term coinbase, but I'd like to give further details on the coinbase transaction.

The coinbase transaction is a special type of transaction.

• Every block must have a coinbase transaction, other transactions are optional.
• The coinbase transaction must be the first transaction of the block (it follows that there can only be one per block).
• The coinbase's output is used to send the block reward, i.e. block subsidy plus transaction fees, to the miner's address.
• The coinbase transaction can have an arbitrary input of 100 byte size. E.g. this was where the genesis block famously contained "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks". This is also the field that may be used for the extranonce.