Nine pages that started everything
In October 2008, someone using the name Satoshi Nakamoto posted a short paper titled *Bitcoin: A Peer-to-Peer Electronic Cash System*. The goal was almost stubbornly simple: let two people send money directly, online, without a bank or payment company sitting in the middle to approve it. For decades that had seemed impossible — remove the trusted middleman and what stops someone from spending the same coin twice?
Bitcoin's answer was to let *everyone* keep the same public record. Instead of one bank's private ledger, there is a shared history that thousands of computers hold copies of and agree on through proof of work. Once a payment is buried under enough later blocks, rewriting it would cost more energy than anyone could sanely spend. The result is electronic cash that nobody owns and nobody can quietly edit.
The ledger has no balances
Here is the surprise. If you opened Bitcoin's real database, you would find no row that says "this address owns 2.5 BTC." There is no account, no stored balance, anywhere. What the network actually keeps is a giant set of coins that have been created but not yet spent — each one a discrete chunk of value locked to a specific owner. This set is called the UTXO set, short for *Unspent Transaction Outputs*.
The cash analogy is exact. Think of a UTXO as a physical banknote sitting in your wallet — a fixed denomination you either spend whole or not at all. Your "balance" is not a number stored anywhere; it is simply the sum of every unspent note your keys can unlock. Your wallet app totals them up for you, but the chain itself only knows about the individual notes.
How a transaction reshapes the set
A Bitcoin transaction is just a recipe that destroys some old notes and prints some new ones. It names a list of existing UTXOs as its inputs — the notes being spent — and defines a list of brand-new UTXOs as its outputs — the notes being created. The moment the transaction confirms, the inputs are deleted from the UTXO set forever, and the outputs are added in their place.
Because notes can't be torn in half, you usually overpay and hand yourself the difference. If you owe 0.7 BTC but your smallest usable notes are 0.5 and 0.4, you spend both (0.9 total), send 0.7 to the recipient, and create a fresh note of just under 0.2 paid back to your own address — the famous *change* output. The tiny slice you leave behind becomes the miner's fee. Every input must be unlocked with a valid digital signature from the owner's key, which is what proves you are allowed to spend those particular notes.
TRANSACTION (Alice pays Bob 0.7 BTC)
INPUTS (old UTXOs, now destroyed)
utxo_A : 0.5 BTC <- unlocked by Alice's signature
utxo_B : 0.4 BTC <- unlocked by Alice's signature
total in = 0.900
OUTPUTS (new UTXOs, now created)
-> Bob : 0.700 BTC
-> Alice : 0.199 BTC (change)
total out = 0.899
fee to miner = in - out = 0.001 BTC (never written down; it's the gap)
Rule: sum(inputs) >= sum(outputs). The leftover is the miner fee.Where the very first coins come from
If every note must trace back to an earlier note, there is an obvious puzzle: where did the *first* notes come from? Bitcoin solves this with one special exception per block — the coinbase transaction. It is the only transaction allowed to have no inputs at all. Instead of consuming old UTXOs, it simply mints new coins out of thin air and pays them to the miner who built the block.
This coinbase output is the block reward — the incentive that makes proof of work worth doing. It bundles two things: a fixed number of newly created coins (which halves roughly every four years) plus all the fees from the other transactions in that block. So every bitcoin in existence was born as some miner's reward, then flowed outward, note by note, through ordinary transactions ever since.
COINBASE TRANSACTION (one per block, no inputs)
INPUTS
(none) <- the special exception
OUTPUTS
-> Miner : block_subsidy + sum(all fees in block)
e.g. 3.125 newly minted BTC + 0.18 BTC collected in fees
= 3.305 BTC paid to the block's minerThe takeaway
Satoshi's 2008 design fits together with surprising economy. Money is a set of unspent notes, not a table of balances. A transaction consumes whole notes as inputs and prints new ones as outputs, change included, with signatures proving ownership and a no-free-money rule keeping it honest. And new coins enter the world only through the coinbase transaction, which rewards the miner who secured the block with proof of work.
Hold on to one image and the rest follows: Bitcoin is a pile of banknotes, not a bank account. Every payment tears up old notes and prints exact replacements. Next, we will zoom into how those notes get *locked and unlocked* — the small stack-based language, Bitcoin Script, that decides what it takes to spend a coin.