Custody: who really holds your bitcoin

Ownership is a key, not an account

There is no database row anywhere that says a coin belongs to you. There are outputs on the chain, each locked to a condition, usually 'a signature from this key'. If you control the key, you can move the output. If you do not, you cannot, and nobody can move it for you.

This is why 'not your keys, not your coins' is a factual statement rather than a slogan. When an exchange holds your bitcoin, the chain shows the exchange's key controlling the output. What you own is a promise from a company, which is a different kind of asset with a different kind of risk.

That trade-off is legitimate in both directions. A custodian can reset your password; a self-custody mistake is final. Choose deliberately, and know which one you have chosen.[3][8]

How a seed phrase actually works

A BIP39 phrase is a human-readable encoding of random entropy plus a checksum. From that entropy, BIP32 derives a whole tree of private keys, which is why one backup can restore every address a wallet has ever used.

Two consequences follow. First: the words are the money. Anyone who reads them can spend, and no software update, support desk or court order changes that. Second: the words are enough. You do not need a backup of the app, only of the phrase and the derivation details.

This site never asks for a seed phrase or a private key, and no legitimate service ever will. Any page, message or 'support agent' asking for your words is an attempt to take your coins.[7][8][30]

Failure modes, ranked by how often they bite

Lost backup. The single most common way people lose bitcoin is not theft but a phrase that was never written down, was written down once on paper that got thrown away, or was stored in a photo on a phone that died.

Phishing and fake support. Attackers do not break cryptography; they ask you politely. Fake wallet apps, cloned exchange pages and 'validate your wallet' messages are the working attacks in practice.

Custodian failure. Companies that hold other people's coins have failed before, sometimes because they lent them out. A balance on a screen is not the same as an output on the chain.

Over-complication. A setup so clever that your heirs cannot follow it is a failure mode too. Write down the recovery procedure in plain language, and test it.[30]

A workable checklist

Start small: move an amount you would not mind losing, and practise sending and receiving before anything larger.

Write the phrase on paper or metal, store it somewhere a flood or a housemate will not reach, and never type it into a website, chat or photo.

Verify addresses on the device that holds the key, not only on the screen that asked for the payment.

Restore from your backup into a fresh wallet at least once, then send a small amount to confirm you can actually spend.

Write a one-page instruction for someone you trust, describing where the backup is and what to do, without putting the words themselves in that document.[30][3]