In this lesson you will learn to:
- Describe the UTXO model.
- Explain why Bitcoin's scripting is deliberately limited.
- Describe how the issuance schedule and rule changes work.
A system with one job
Bitcoin was the first working public blockchain, launched in January 2009. Its design is deliberately narrow: it records the transfer of one kind of unit, and it changes very slowly. Understanding it as a system makes every later design easier to follow.
No accounts: unspent outputs
A bank statement shows a balance. Bitcoin's record does not store balances at all. Instead, every transaction consumes earlier outputs and creates new ones. An output that has not yet been used is called an unspent transaction output, or UTXO.
A helpful comparison is paper notes. To pay with a note larger than the amount, you hand over the whole note and receive change. A Bitcoin transaction works similarly: it uses up whole outputs and creates new ones — one for the recipient and often one returning the remainder to the sender, as "change".
What wallet software shows as a balance is simply the sum of the UTXOs its keys can sign for. The double-spending check from lesson 1.1 becomes concrete: every node checks that each output being used exists and has not been used before.
A small, cautious scripting language
Each output carries a short condition written in a simple language called Script: typically, "can be used by whoever provides a valid signature for this public key". Script can express a few more conditions — several signatures, time locks — but it is deliberately not a general programming language. It has no loops, which makes every script predictable and cheap to check.
A fixed issuance schedule
New units enter circulation only as the incentive for adding blocks (lesson 3.3). The amount created per block halves roughly every four years (every 210,000 blocks), so the total number of units that can ever exist approaches 21 million. This schedule is written into the rules every node enforces. Whether it is a good monetary design is a debate among economists; for this course, what matters is that it is enforced by software running on many independent computers.
How rules change
Because every node enforces the rules, changing them requires broad agreement. Proposals are published as Bitcoin Improvement Proposals (BIPs), discussed in public, and activated only if enough of the network adopts them. Changes designed so that old nodes still accept new blocks are called soft forks; the 2021 "Taproot" upgrade, which added a new signature scheme, is an example. This slow, cautious process is a deliberate trade-off: stability over speed.
Building on top
Because the base layer changes slowly, developers build additional systems on top of it — for example, payment channels that settle many small transfers between two parties and record only the final result on the main chain. These "layer 2" designs are an active area of engineering across many blockchains.
Key takeaways
- Bitcoin records unspent transaction outputs (UTXOs), not account balances.
- Its scripting language is deliberately limited so every rule is predictable.
- New units follow a fixed schedule that every node enforces.
- Rule changes need broad agreement and are adopted slowly by design.
Check yourself
What does a Bitcoin transaction do with UTXOs?
It uses up existing unspent outputs in full and creates new outputs, often including change back to the sender.
Why does Script have no loops?
To keep every script predictable and cheap for every node to check.
What is a soft fork?
A rule change designed so that older nodes still accept blocks created under the new rules.
Sources
This lesson is for general education about how the technology works. It is not a recommendation to acquire, sell or hold any crypto-asset and is not financial advice.