A deterministic tree instead of unrelated keys
Older wallets could generate many independent random keys, so a backup also had to capture newly created keys. BIP 32 describes a hierarchical deterministic wallet: it derives a tree of parent and child keys from one root seed. The same root and the same derivation rules produce the same keys.
An extended public key can let a watch-only wallet derive receiving addresses without signing. Revealing it can expose the history and balances of an entire branch, however, so it is not just an ordinary public address to share casually.
What BIP 39 does
BIP 39 encodes 128 to 256 bits of computer-generated entropy plus a short checksum as 12, 15, 18, 21 or 24 words. The words are not a random sentence invented by a person. Their order, exact spelling and word list are part of the data.
A defined function turns the mnemonic and an optional passphrase into a 512-bit seed that BIP 32 can use. BIP 39 is an application standard, not a Bitcoin consensus rule, and not every wallet uses it. Word count alone therefore proves neither compatibility nor implementation quality.
- the words represent computer-generated randomness and a checksum
- the same words with a different passphrase produce a different seed
- the seed is the root; individual addresses result from further derivation
Why correct words can show an empty wallet
Wallets use derivation paths and different output types, such as legacy, native SegWit or Taproot. If recovery software searches a different branch or interprets the seed differently, it can show a zero balance even though the keys are not lost. The account, network and descriptor may also matter.
A backup plan should therefore record the wallet or standard, address type, any passphrase and suitable technical details—but not combine them in a way that gives every finder everything immediately. Recovery should follow the documentation of compatible software.
A seed is spending power
Anyone who obtains the seed and required passphrase can derive the keys on their own device and sign a transfer. They do not need the original hardware or the manufacturer's permission. A seed should never be photographed, sent in a message or entered into a website offering a check or support.
A checksum detects some typing errors, not theft or a weak human-created sentence. A trusted wallet should generate the seed from good randomness. Self-selected words or an online generator can create a predictable or already exposed root.