Open your bitcoin wallet. You see a balance, say 0.01 BTC. Your app calculated that number itself. It isn't written anywhere on the blockchain.

What the blockchain $BTC keeps are pieces. Each payment you receive creates a piece in your name, with a fixed amount. Developers call this a UTXO, short for “unspent transaction output.” Put simply: a bill you haven't used yet. Your “balance” is just the sum of all those bills.

And a bitcoin bill can't be split. You spend it all, or not at all.

A TRANSACTION, TAKEN APART PIECE BY PIECE

Before. You received three payments: 0.005, 0.003, and 0.002 BTC. Three bills, totaling 0.01.

During. You want to send 0.006 BTC. Your wallet picks bills that cover the amount, for example, 0.005 + 0.003 = 0.008. It “breaks” both of them. Then it creates new bills: 0.006 for the recipient, and a change bill that comes back to you, for example, 0.00195.

After. There’s 0.00005 BTC missing between what went in (0.008) and what went out (0.00795). That gap is the fee. There’s no “fee” field in a bitcoin transaction: the miner takes whatever hasn’t been allocated.

THREE WEIRD THINGS THIS EXPLAINS

1. Fees don’t depend on the amount. Sending $10 or $10,000 can cost the same. What matters is the transaction size in bytes, which mostly depends on how many bills you break. Spending twenty small bills costs more than spending one big one.

2. An unfamiliar address appears in your history. That’s often your change. Many wallets send it to a new address that belongs to you, so your transaction history isn’t exposed to everyone. It’s not a hack.

3. Some small amounts get “stuck.” A bill worth a few hundred satoshis (a satoshi is one hundred-millionth of a bitcoin) can cost more in fees than it’s worth. This is called dust.

THE DAY THE CHANGE GOT LOST

On September 10, 2023, a transaction moved about 0.074 BTC, roughly $2,000. The fee paid: 19.82 BTC, worth more than $500,000 at the time. The sender was Paxos, which said there had been a bug in its internal operations. Jameson Lopp, a well-known bitcoin developer, suggested one possible explanation: the change output may have been calculated incorrectly. The remainder went to the miner, F2Pool, which eventually refunded the whole amount on September 15 after an investigation. An act of goodwill, not an obligation.

I find this more compelling than any diagram. Forgetting to give yourself your change on bitcoin means giving away the difference.

AND WHAT ABOUT ETHEREUM?

$ETH works differently: each account has an actual balance, a number that goes up or down, like a bank account. No bills, no change. Fees (“gas”) depend on the work requested from the network. Two designs, two philosophies: bitcoin optimizes for simple verification of each coin, while Ethereum prioritizes the flexibility of accounts and contracts.

Useful note: on an exchange, your internal balance isn’t made up of bills. UTXOs only come into play when you withdraw to a wallet you control, and the exchange puts the transaction together.

MY TAKE

If you receive lots of small bitcoin payments, for example, regular transfers of a few thousand CFA francs, you accumulate small bills. That’s no problem until the day you want to move them all during a fee spike. Reputable wallets let you combine these bills into one when the network is quiet. It’s not required, but it’s the kind of detail that separates people who get stuck paying fees from those who choose when to pay them.

Question for you: does your wallet show you a list of your bills (often called “coin control”), or just the total?

Sources: Bitcoin developer documentation (UTXO model), CoinDesk, The Block, and CryptoSlate (Paxos/F2Pool incident, September 2023), Ethereum documentation (account model).

This is not financial advice.

#Bitcoin #UTXO #Frais #Pedagogie #Ethereum