What a seed phrase actually is
A BIP39 mnemonic is a human-readable encoding of 128–256 bits of entropy: your machine generated random bits, a checksum was appended, and the bits were sliced into 11-bit chunks, each mapped to one word from the 2048-word list. 12 words = 128 bits entropy (4-bit checksum); 24 words = 256 bits (8-bit checksum). The words carry no meaning — word 4 isn't "the fourth part of your wallet" — the phrase is just a number, encoded so humans can transcribe it without making errors.
From words to keys
The mnemonic is run through PBKDF2-HMAC-SHA512 with 2048 iterations and the passphrase "mnemonic" + passphrase to produce a 512-bit seed. That seed feeds BIP32 to derive the master key, and BIP44 defines the standard derivation paths (m/44'/60'/0'/0/0 for the first Ethereum address) — which is why the same phrase restores the same addresses in every compliant wallet.
The passphrase ("25th word")
BIP39 lets you add a passphrase that changes the derived seed entirely: same words + different passphrase = completely different wallet, with no way to detect one from the other. This enables plausible deniability ("decoy wallet under the standard passphrase"), but it's also the #1 way people lose funds — a passphrase you forget makes the phrase useless. Unlike the words, the passphrase is not restricted to any list and is case-sensitive.
Security practices
- Never type a mnemonic into a website. Hardware wallets display the phrase for a reason: it must exist only on your device and paper. Any page asking for it is a drainer.
- Test restoration with a small amount before funding — a transcription error should be caught early, not at recovery time.
- Checksums matter: an invalid word order usually fails the checksum loudly, but a wrong-but-valid phrase restores a valid empty wallet — which is the failure you won't notice until it matters.
- Entropy source: generate with a hardware wallet or vetted software; "made up" words have a fraction of the entropy and are crackable.
Understand and experiment with derivation safely (offline, test values only) in the BIP39 Passphrase tool — everything runs locally in your browser.