An order book is a list of orders that a trading venue uses to record the orders posted by “Makers”, wishing to trade. Shark Relay hosts an Order Book where Maker's post their orders cryptographically signed to ensure security which are then taken by a “Taker”. The Order Book is hosted ”off-chain” which means the bulk of transactional data is stored separate from the Ethereum Blockchain and only final settlement is recorded to the Ethereum Blockchain. This is done to allow high transaction volumes in real time at a low cost.