I've received many questions from people considering to set up a #Bitcoin multisig wallet but confused about the backup process, what should be backed up, and why.
🧵So here's a thread on backing up multisig wallets - what, why, and how.
The main caveat in a multisig wallet is that, while you need only a threshold of devices (ie. 2 of 3, 3 of 5, etc.) to sign a transaction, losing access to even a single device could potentially prevent you from being able to spend the funds - if you don't back up properly!
The reason is that (usually) in order to make a Bitcoin transaction, it is not enough to be able to sign it, but you also need to provide the "terms for spending", that is, the Script (code) that is used to lock the coins.
That's because the full script is usually not stored directly on the blockchain, but only as a hash (a unique identifier of it).
Then on spending, you need to provide both the original script code which matches that hash, and satisfy any condition that the script may include.
In a single key wallet, this is trivial.
The script is composed of the public key of your wallet and a few standard commands - and satisfying it requires the signature for that key, so both the script and signature can be constructed by having access to that one private key.