What happens when you add a passphrase to a seed phrase?
Correct answer
Answer A: Each different passphrase produces a different wallet.
Quick explanation
A passphrase is an additional password used with the seed phrase. If you forget it, you cannot restore that wallet using the seed phrase alone.
Detailed explanation
The optional BIP39 passphrase is not a normal password that merely encrypts an existing wallet. During seed calculation, the mnemonic is processed with the string mnemonic plus the passphrase. Changing even one character produces a different seed and therefore a different hierarchy of keys and addresses. Every possible passphrase is technically valid. A wallet cannot therefore reliably report that an entry was wrong; it often simply displays a different, empty wallet.
The common name 25th word is misleading. The passphrase need not be a word from the BIP39 list and can also be used with twelve words. It can increase security if mnemonic and passphrase are protected separately, but it also substantially increases the risk of user error. If the passphrase is lost or conveyed inaccurately, the mnemonic is not enough to recover the intended wallet. Adding a passphrase later does not change old addresses; bitcoin must be transferred to the newly derived wallet. This feature therefore belongs in experienced, tested procedures.
Example or everyday application
With the same mnemonic, the passphrases Segel and segel create two different wallets. Neither is marked wrong; only the exact original entry opens the expected addresses.
Common misconception
The passphrase is often seen as a resettable device password or mandatory 25th word. It actually changes seed derivation; even an unnoticed typo leads to a different wallet.
Sources used
BIP 39 - Mnemonic code for deterministic keys
This source is mainly intended for developers. The relevant information may therefore be harder to find.
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