Bank reserves or central bank reserves are banks' holdings of deposits in accounts with their central bank (for instance the European Central Bank or the Federal Reserve, in the latter case including federal funds), plus currency that is physically held in the bank's vault ("vault cash"). Some central banks set minimum reserve requirements, which require banks to hold deposits at the central bank equivalent to at least a specified percentage of their liabilities such as customer deposits. Even when there are no reserve requirements, banks often opt to hold some reserves —called desired reserves— against unexpected events such as unusually large net withdrawals by customers or bank runs.