PSBT
Also: Partially Signed Bitcoin Transaction, BIP-174
wallets · intermediate
A standardized transaction format that lets multiple parties — or an air-gapped device — sign in stages, without exchanging private keys.
BIP-174 (2017) standardized how an unsigned or partially-signed transaction is represented as a binary blob. A wallet builds the inputs, outputs, and metadata; a separate signer fills in signatures; a third party can combine signatures from a multisig; a finalizer produces the broadcast-ready hex.
PSBT is what makes air-gapped signing practical. A watch-only wallet on an online laptop constructs the transaction and exports it via QR or SD card. A cold-storage device (Coldcard, BitBox, Trezor, Specter-DIY) imports it, signs, and exports back. The private keys never touch the internet.
The format also lets multisig coordinators (Nunchuk, Specter, Sparrow) collect signatures from several devices without any of them seeing the others' keys. Each signer only needs the PSBT and their own seed.