An off-chain order book is made possible by “Relayers,” services that relay messages (known as broadcast orders) between participants off-chain. A participant (Maker) can submit a cryptographically signed order to a Relayer. If the order is valid, the Relayer will post it to the order book in return for a transaction fee, which is only paid if the order is filled. Once the order book is updated, any participant (Taker) can choose to fill the Maker’s order.

A Taker can fill a Maker’s order by sending a cryptographically signed message to a DEX smart contract. If valid, the DEX will execute the trade between the Taker and Maker on-chain, transferring tokens between the participants.

Relayers do not play a role in trade execution, they simply facilitate communication between Makers and Takers. Because Takers must send a cryptographically signed message to a DEX in order for the DEX to execute a trade, they have final say over trade execution and do not need to trust Relayers, resulting in trustless trading.

ZRX is the digital asset token used to pay Relayer fees and participate in the governance of the 0x protocol. It is built on Ethereum in accordance with the ERC20 standard for tokens. ZRX can be bought and sold for fiat currency or other digital currencies.

ZRX can be stored in a crypto wallet and custodian like Gemini.