Foreign exchange reserves assets can comprise banknotes, deposits and government securities of the reserve currency, such as bonds and treasury bills. Some countries hold a part of their reserves in gold, and special drawing rights are also considered reserve assets. Often, for convenience, the cash or securities are retained by the central bank of the reserve or other currency and the "holdings" of the foreign country are tagged or otherwise identified as belonging to the other country without them actually leaving the vault of that central bank. From time to time they may be physically moved to the home or another country.