Crypto glossary
Shamir backup (SLIP-39)
A Shamir backup (SLIP-39) splits a wallet's recovery secret into several word shares, and only a set threshold of them, such as 2 of 3, restores it.
Shamir's Secret Sharing, published by cryptographer Adi Shamir in 1979, divides a secret into parts so that any chosen number of them can rebuild it while fewer reveal nothing. SLIP-39 applies the scheme to wallet backups: instead of one seed phrase, the wallet produces several shares of 20 or 33 words, each labelled with the threshold needed. Advanced setups can nest groups of shares with their own thresholds.
The appeal is resilience without a single point of failure. A 2-of-3 backup lets one share be lost or stolen without losing or exposing the funds, and shares can be spread across a home safe, a bank box and a trusted relative. Trezor popularised the standard in its hardware wallets, and it is common in inheritance and family custody plans.
Shamir is not multisig. The full secret is recombined on one device during recovery, and the wallet still signs with a single key, so that device must be trusted. SLIP-39 shares are not compatible with BIP-39 seed phrases or with every wallet, and homemade splitting of a normal phrase does not give the same guarantees.
Example
A family sets up a 2-of-3 Shamir backup, keeping one share at home, one in a bank deposit box and one with their lawyer, so any two can restore the wallet.