The OutPoint: TXID and VOUT

To identify a specific piece of bitcoin in the multi-terabyte history of the blockchain, Bitcoin uses a 36-byte structure called an OutPoint.

1. Defining the OutPoint

An OutPoint is the unique coordinate of a UTXO. It consists of two pieces of data:

1. TXID (32 Bytes): The unique ID of the transaction that created the output.

2. VOUT (4 Bytes): The index of the output within that transaction (starting at 0).

2. Why Two Fields?

A single transaction can have dozens or even hundreds of outputs.

• If you only provided the TXID, the network wouldn't know which specific output you were spending.

• The VOUT (Output Index) acts as the specific "slot" number.

3. Storage in the Input

In the raw hex of a transaction, the OutPoint is the very first thing listed for each input.

• It is stored in Little-Endian.

• This means the 32-byte TXID and the 4-byte VOUT are both reversed in the raw binary data.

4. The Genesis Case (Coinbase)

The only time an input does not point to a previous transaction is in a Coinbase Transaction.

• In a coinbase input, the TXID is 32 bytes of zeroes (0000...).

• The VOUT is 0xffffffff (4.2 billion). This special OutPoint signals to nodes that this transaction is creating new bitcoin, rather than spending old bitcoin.

Component	Format	Size
TXID	Double-SHA256 Hash	32 Bytes
VOUT	Unsigned Integer	4 Bytes
Total	Raw Coordinate	36 Bytes

[!NOTE] Every full node maintains a "UTXO Set" (Chainstate) indexed by OutPoints. This allows the node to verify an input instantly without searching the entire blockchain.

In the next section, we will explain how the TXID itself is calculated.