Merkelized Alternative Script Tree (MAST)

A Merkelized Alternative Script Tree, or MAST, is a way to encode Bitcoin spending conditions as leaves of a Merkle tree, so that only the condition actually used is ever revealed on chain. The idea circulated in proposals such as BIP 114 and BIP 117 before its core concept was folded into Taproot, which activated in November 2021.

Why it matters

MAST shrinks transaction size, since a contract with dozens of clauses costs little more to spend than one with a single clause, and it improves privacy, because observers never learn what the untaken paths were. Complex ownership arrangements, such as inheritance plans and multi-party vaults, become cheap and private on Bitcoin. Replicating equivalent conditional ownership with physical gold requires lawyers, custodians and trust in enforcement rather than mathematics.

How it works

A script with several alternative spending paths, for example a normal signature path plus an emergency timelock path, is split into separate leaf scripts. The leaves are hashed into a Merkle tree and only the root is committed on chain. To spend, the owner reveals just one leaf plus a short Merkle proof, keeping every unused branch private and off chain. Taproot's script tree implements this design and adds a key path that hides the tree entirely when participants cooperate, so a cooperative spend looks like an ordinary single-signature payment.