Users can transact directly without an intermediary. Transactions are verified by network nodes and recorded in a public distributed ledger called the block chain. The ledger uses bitcoin as its unit of account. The system works without a central repository or single administrator, which has led the U.S. Treasury to categorize bitcoin as a decentralized virtual currency. Bitcoin is often called the first cryptocurrency, although prior systems existed. Bitcoin is more correctly described as the first decentralized digital currency. It is the largest of its kind in terms of total market value. Bitcoins are created as a reward for payment processing work in which users offer their computing power to verify and record payments into a public ledger.