The 13th word, and what happens if you forget it         

The twelve words derive your keys. A passphrase — the "13th word" — is a second secret you choose yourself, mixed in before the keys are derived.

Its point is that a phrase somebody finds is useless without it. Twelve words on a sheet of paper in a drawer stop being a bearer instrument.

Its cost is exactly as large.

The passphrase is never stored — not by the app, not on our side, not in the backup. It exists only where you remember it.

What forgetting it looks like

Not an error. The twelve words plus a wrong passphrase derive a valid wallet — a different one, with different addresses and no funds in it. The app cannot tell you that you typed the wrong thing, because from the mathematics both answers are correct.

That is why a passphrase you cannot recall under stress is worse than no passphrase at all.

If you use one

• Choose something you will still produce in five years, in the same spelling, spacing and case.
• Back it up separately from the twelve words, in a different place. Together on one sheet, they are one secret again.
• Test the whole thing before funding: restore the wallet from the phrase and the passphrase, and confirm the addresses match.