Keys, addresses, and UTXOs

Almost everyone starts with the same mental picture: a wallet is a container, bitcoin sits inside it, and sending bitcoin moves it from your container to someone else's. It is a reasonable guess and it is wrong in every part, which is why so much of bitcoin seems arbitrary until you replace it.

This page builds the correct picture from the bottom up. It takes about fifteen minutes and it makes everything afterwards easier — why fees behave oddly, why your wallet keeps generating new addresses, why you can restore everything from twelve words, and why sending a small amount sometimes moves your entire balance.

A clean overhead flat-lay: a key, a padlock, and a small pile of mismatched cash notes of odd denominations, arranged left to right.

Image to come

1There are no coins

Nothing is stored in your wallet. There is no file containing bitcoin, and nothing physically moves when you send a payment.

What exists is a public ledger — a record, copied across thousands of computers, of every transaction ever made. That ledger does not track balances by person. It tracks amounts and the conditions required to spend them. Somewhere in it are entries saying, in effect, "this much bitcoin, spendable by whoever can prove they hold a particular key."

Your wallet's job is to hold that key and to prove it, on demand, without ever revealing it. That is the entire trick.

2The private key is a number that can sign

A private key is, underneath, an enormous random number. Its usefulness is that it can produce a signature: a piece of data that anyone can check, that could only have been produced by that key, and that reveals nothing about the key itself.

That last part is what makes bitcoin work. You prove you are allowed to spend without ever handing over the thing that allows it. Every payment you make is a signature saying "the holder of this key authorises this exact transaction" — and if a single detail of the transaction changes, the signature no longer matches.

3Addresses are a one-way street

From your private key, your wallet derives a public key, and from that, an address. The derivation only runs one way: address from key is easy, key from address is impossible.

Think of an address as a padlock you can hand out freely. Anyone can snap it shut on a payment. Only your key opens it.

That one-way property is why publishing an address is safe. It is also why an address is not a wallet, not an account, and not a login — it is a destination, and one wallet can produce an effectively unlimited number of them.

4One seed, unlimited keys

If a wallet needed a separate backup for every key, self-custody would be unmanageable. It does not, because the keys are not independent.

Your twelve or twenty-four recovery words encode a single starting number: the seed. From that seed, your wallet derives every key it will ever use, in a fixed and standardised order. Key number one, key number two, key number five hundred — all reproducible, from those words, forever.

• This is why the words are the backup, and the only backup that matters.
• This is why you can restore the same wallet in different software and see the same coins.
• This is why anyone who has the words has everything, immediately and permanently.
• And it is why the words can be written on paper: they encode one number, and that number regenerates the rest.

5Why your wallet keeps making new addresses

You may have noticed that asking for a receive address twice gives you two different addresses, and that the old one still works. That is deliberate.

Because the ledger is public, anyone who knows one of your addresses can see every payment it ever received. Reusing a single address builds a public, permanent record of your income in one convenient place. Using a fresh address each time scatters that information instead.

It is a privacy measure rather than a security one — an old address is not unsafe, it is just revealing. All of them belong to the same wallet, and your wallet watches all of them.

6Your balance is a pile of chunks

Here is the idea that surprises people most, and the one that explains the most.

The ledger does not store "you have 0.5 bitcoin." It stores individual, indivisible outputs — each one created by a specific past transaction, each with its own amount. The unspent ones are called UTXOs: unspent transaction outputs. Your balance is simply the sum of yours.

The closest everyday comparison is cash, but cash in strange denominations. If someone paid you 0.3, someone else paid you 0.15, and you bought 0.05, you do not have "0.5." You have a 0.3 note, a 0.15 note, and a 0.05 note. Your wallet adds them up and shows one number, which is a convenience, not the truth.

7What follows from all this

Almost every piece of bitcoin behaviour that seems arbitrary at first is a direct consequence of the four ideas above.

That fifth row is worth pursuing once the rest has settled. It is the entire basis of coin control and labelling, and it is the difference between a wallet that quietly assembles a public map of your finances and one that does not.

The short version

• You own keys, not coins. Ownership is the ability to sign.
• Addresses are one-way and safe to share. Use a fresh one each time.
• One seed makes every key, which is why the recovery words are the whole backup.
• Your balance is a set of chunks, and spending one always spends all of it, with the remainder returning as change.

If those four sentences make sense, you have the model. Everything else on this site is detail hung on that frame.