The system is peer-to-peer and transactions take place between users directly, without an intermediary.[15]:4 These transactions are verified by network nodes and recorded in a public distributed ledger called the block chain,[16] which uses bitcoin as its unit of account. Since the system works without a central repository or single administrator, the U.S. Treasury categorizes bitcoin as a decentralized virtual currency.[1] Bitcoin is often called the first concurrency,[17][18][19] although prior systems existed[note 5] and it is more correctly described as the first decentralized digital currency.[15][22] Bitcoin is the largest of its kind in terms of total market value
