FOLLOW THE MECHANISM
- 1Select inputs
Choose existing unspent outputs.
- 2Satisfy conditions
Provide signatures or other required data.
- 3Create outputs
Allocate recipient value and change.
- 4Pay fee
Leave the input/output difference to the block producer.
A conceptual sequence. Real implementations can add steps, intermediaries, and conditions.
An output is a future spending condition
A transaction output specifies an amount and the conditions under which it can later be spent. While unspent, it belongs to the UTXO set. A later transaction references that output and supplies the required authorization. The old output is consumed in full; the new transaction can distribute its value across several replacement outputs. This is why a wallet balance is an aggregation rather than one editable coin container.
Change is part of the transaction
If selected inputs exceed the intended payment and fee, the sender typically creates another output under its own control. That output is change, not a refund sent later by the recipient. A wallet chooses which outputs to consume and where to send change. The public transaction shows the full input and output structure, though an observer may not know with certainty which new output is the payment and which is change.
Fees reflect resource demand
For an ordinary Bitcoin transaction, the fee equals total input value minus total output value. Size and script structure affect the resources a transaction uses and the fee rate needed for inclusion. Spending many small outputs can require more transaction data than spending one larger output of equal total value. The monetary value transferred alone is therefore not a complete explanation of the fee.
Parallel proposals can conflict
Two transactions can each reference the same unspent output, but both cannot remain valid in one accepted history. Once one consumes it, the other lacks an unspent input. A pending wallet display is consequently not final evidence that a payment is settled. Output selection also affects privacy: combining outputs can reveal relationships between previously separate addresses, while reusing address patterns can make tracing easier.
MAKE IT CONCRETE
Conservation in a payment
Illustrative transaction: inputs are 0.6 and 0.5 example BTC, totaling 1.1. A recipient output is 0.7, change is 0.399 and the fee is 0.001. The equation is 1.1 = 0.7 + 0.399 + 0.001. Both original outputs disappear from the UTXO set; the recipient and change outputs become new spendable entries.
FAILURE PATHS & COMMON MISREADINGS
Where the model can break
KEEP EXPLORING