Bitcoin transactions are permanently recorded in a public distributed ledger called the block chain. Approximately six times per hour, a group of accepted transactions, a block, is added to the block chain, which is quickly published to all network nodes. This allows bitcoin software to determine when a particular bitcoin amount has been spent, a novel solution for preventing double-spends in a peer-to-peer environment with no central authority. Whereas a conventional ledger records the transfers of actual bills or promissory notes that exist apart from it, the block chain is the only place that bitcoins can be said to exist.[23] To independently verify the chain-of-ownership of any and every bitcoin amount, full-featured bitcoin software stores its own copy of the block chain.
