Stacks (STX) is a layer 2 blockchain for Bitcoin designed to enable the creation of smart contracts and decentralized applications (dApps) on the network, without modifying the underlying protocol. The technology of Stacks is called Proof of Transfer (PoX) and consists of a mechanism for compressing and recording information from Stacks blocks in the OP_RETURN function of Bitcoin transactions, thereby ensuring the immutability and security of Stacks transactions.