Seed Phrase

A seed phrase is a sequence of 12 or 24 English words that encodes the master secret for an entire Bitcoin wallet. Also called a "mnemonic phrase" or "recovery phrase" - the terms are interchangeable in practice.

What it is, mechanically

The standard is BIP-39, published in 2013. It defines:

• A fixed list of 2,048 words. Each word is unique in its first four letters, so a half-typed word still resolves unambiguously and transcription errors are easier to catch.
• A mapping from binary entropy to word sequences. Each word encodes 11 bits.
• A SHA-256-derived checksum, so a mistyped or randomly-guessed phrase fails validation instead of silently opening a different (and probably empty) wallet.
• A derivation from words to a 512-bit master seed: PBKDF2-HMAC-SHA512, 2,048 iterations, with an optional user-chosen passphrase as the salt.

That 512-bit master seed feeds into BIP-32 hierarchical key derivation, which produces every private key and address the wallet uses. The chain - words to master seed to master private key to derived keys to addresses - is purely deterministic. Same words in, same wallet out, on any BIP-39 + BIP-32 compatible software.

12 words encodes 128 bits of entropy. 24 words encodes 256 bits. Either is more than enough; 128 bits of randomness is unguessable in any practical sense, and 256 bits is well beyond the reach of any conceivable adversary.

Why it has won

Portability. Back up with one wallet, restore on a completely different brand, your coins are still there - as long as both implement the standard. That cross-vendor interoperability is one of the quiet wins of Bitcoin's standards process.

The phrase is the complete backup. From those 12 or 24 words, a hierarchical deterministic wallet can regenerate every private key, every public key, and every address the wallet has ever used or will ever use. Lose your hardware wallet, your phone, your laptop - all of it - you can restore the wallet on any compatible device by typing the words back in.

That power cuts both ways. Anyone who reads your seed phrase has the same capability.

Storage rules

• Write it down on paper or metal. Do not photograph it. Do not type it into a computer except into a wallet during restore. Do not store it in a password manager or cloud drive.
• Verify the backup. Practice restoring on a fresh device. A backup you have never tested is a backup you do not have.
• Consider geographic redundancy. A house fire that destroys your seed and your hardware wallet has destroyed your money. Two or three copies in physically separate locations is reasonable for non-trivial amounts.
• Consider a passphrase (the "25th word"). An optional, user-chosen additional secret that combines with the 12 or 24 words to derive a separate wallet. Powerful but adds operational risk: forget the passphrase and the funds are gone.

The seed phrase is the one thing you must protect to truly self-custody Bitcoin. Everything else is recoverable; this is not.

See the Seed Backup Strategies rabbit hole for the deeper treatment - paper versus metal, multisig backup schemes, inheritance planning, and how to verify a backup without exposing the words to anyone else.