Backing up a seed phrase properly

Why this lesson exists

Roughly a fifth of all bitcoin is estimated lost, mostly through lost keys. A hardware wallet can be replaced; the seed cannot. The backup is the bitcoin.

Steps

1. 01Write the words in order, numbered, in clear capitals. Check each against the BIP 39 word list; the first four letters of each word are unique, so a smudge is survivable but a wrong word is not.
2. 02Verify by restoring: on a reset device or a second wallet app, enter the words and confirm the same addresses appear. Do this with a small balance before it matters.
3. 03Move to metal. Fire, flood and time destroy paper. Stamped or engraved steel plates survive house fires. Do this once the amount is more than you would be comfortable losing.
4. 04Keep two copies in two locations you control, such as home and a relative's safe. A single copy is a single point of failure; a copy with a third party is a theft risk.
5. 05Never photograph, type, email, cloud-save or screenshot the words. Digital copies can be stolen silently from anywhere in the world.
6. 06Record where the backups are and how to use them in your inheritance plan (lesson 9), without writing the words there.

Passphrases and Shamir

A BIP 39 passphrase adds a word you memorise; without it the seed opens an empty wallet. It protects against a stolen backup and creates a new way to lose everything. Shamir backup splits a seed into shares (any 3 of 5, say). Both are powerful and both add failure modes. Learn them in the Advanced course before relying on them.