Obligations of the U.S. government issued by the U.S. Department of the Treasury as a means of borrowing money to meet government expenditures not covered by tax revenues. All marketable Treasury securities have a minimum purchase amount of $1,000 and are issued in $1,000 increments. There are three types of marketable Treasury securities: bills, notes and bonds. The department also issues several types of nonmarketable securities, including savings bonds.