<p>Each transaction output is uniquely identified by an <a href="https://bitcoin.org/en/developer-reference#outpoint" rel="nofollow noreferrer"><strong>outpoint</strong></a>. Outpoints consist of</p>

<ul>
<li><code>txid</code>: The unique transaction ID of the transaction that created the output. The txid is the 32-byte sha256d hash of the transaction. </li>
<li><code>index</code>: The output index (<code>vout</code>) that marks the position of the output in that transaction. The output index starts counting at zero and is a 4-byte unsigned integer.</li>
</ul>

<p>As an arbitrarily picked example, the outpoint </p>

<pre><code>e06a3193c6ac49b8dd862baf9bd3561ffe86d65b6e8a9e73ebe1dc91e3ca2978:1
</code></pre>

<p>refers to the second output of the transaction <a href="https://www.smartbit.com.au/tx/e06a3193c6ac49b8dd862baf9bd3561ffe86d65b6e8a9e73ebe1dc91e3ca2978" rel="nofollow noreferrer"><code>e06a3193c6ac49b8dd862baf9bd3561ffe86d65b6e8a9e73ebe1dc91e3ca2978</code></a> worth 0.001 BTC which was sent to address <code>1La27vpuDDr9NR1aZ6GpxoJyNYmHT62nRb</code>.</p>

CC BY-SA 3.0
2017-10-31T04:56:36.330Z
First time I heard the word "outpoint". Is it widely used term?
sanket1729
CC BY-SA 3.0
2017-10-31T08:54:05.207Z
@sanket1729: It has been around at least since 2011, see e.g. https://bitcoin.stackexchange.com/a/2359/5406
Murch
CC BY-SA 3.0
2017-10-30T21:18:12.503Z
2017-10-31T09:05:47.600Z
2017-10-31T09:05:47.600Z
Murch
<p>In a bitcoin transaction the previous unspent transaction outputs are referenced.</p>

<p>How are these referenced?</p>

<blockquote>
  <p>Do outputs have unique reference or maybe as</p>
  
  <p><code>unique transaction reference</code> X <code>output recipient's address (its public key)</code></p>
</blockquote>

<utxo>
How does one reference an unspent transaction output?
croraf
# How does one reference an unspent transaction output?

(Asked by: username_0 on 2017-10-30)

In a bitcoin transaction the previous unspent transaction outputs are referenced.

How are these referenced?

> Do outputs have unique reference or maybe as
>
> `unique transaction reference` X `output recipient's address (its public key)`

**Tags:** utxo


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## 1 answers

### Answer by username_1 on 2017-10-30. Score: 5 | Accepted answer

Each transaction output is uniquely identified by an [**outpoint**](https://bitcoin.org/en/developer-reference#outpoint). Outpoints consist of

* `txid`: The unique transaction ID of the transaction that created the output. The txid is the 32-byte sha256d hash of the transaction.
* `index`: The output index (`vout`) that marks the position of the output in that transaction. The output index starts counting at zero and is a 4-byte unsigned integer.

As an arbitrarily picked example, the outpoint

```
e06a3193c6ac49b8dd862baf9bd3561ffe86d65b6e8a9e73ebe1dc91e3ca2978:1

```

refers to the second output of the transaction [`e06a3193c6ac49b8dd862baf9bd3561ffe86d65b6e8a9e73ebe1dc91e3ca2978`](https://www.smartbit.com.au/tx/e06a3193c6ac49b8dd862baf9bd3561ffe86d65b6e8a9e73ebe1dc91e3ca2978) worth 0.001 BTC which was sent to address `1La27vpuDDr9NR1aZ6GpxoJyNYmHT62nRb`.

> **Comments:**
> **username_2**: First time I heard the word "outpoint". Is it widely used term?  
> **username_1**: @username_2: It has been around at least since 2011, see e.g. https://bitcoin.stackexchange.com/a/2359/5406  

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