Definition
Every Taproot output can be unlocked two different ways, and the chooser picks at spending time. The key path is the cheap, private default; the script path is the fallback for richer conditions. This duality is what lets Taproot make complex contracts look ordinary on-chain when everyone cooperates.
Key Path Spend
A key-path spend provides just one witness element: a Schnorr signature for the output key. It is the smallest possible spend and reveals nothing about any alternative conditions that might exist. When all parties to a contract agree, they can use the key path and the transaction is indistinguishable from a simple single-signer payment, even if the key is actually an aggregate of many participants.
Script Path Spend
A script-path spend is used when the cooperative key cannot or should not sign. It reveals one script from the committed tree (a tapleaf), the inputs that script consumes, and a control block proving the script was part of the output. Only the executed branch is disclosed; the rest of the tree stays hidden, so unused conditions never appear on-chain.
The privacy win is structural: an outside observer cannot tell whether a key-path spend came from one signer or a thousand, and cannot enumerate the unused script branches. Cooperative aggregate key-path signing is commonly handled with MuSig2, while the fallback branch relies on the control block (Taproot) to prove membership in the tree.
In Simple Terms
Every Taproot output can be unlocked two different ways, and the chooser picks at spending time. The key path is the cheap, private default; the…