Nothing and/or different notes.
"Federal Reserve Notes are
fiat currency, which means that the government is not obligated to give the holder of a note
gold,
silver, or any specific tangible property in exchange for the note. Before 1971, the notes were "backed" by gold—i.e., the law provided that holders of Federal Reserve notes could exchange them on demand for a fixed amount of gold (though from 1934–
1971 only
foreign holders of the notes could exchange the notes on demand).<sup class="reference" id="_ref-0">
[1][/SUP] Since 1971, federal reserve notes have not been backed by any specific asset. While
U.S.C. 12,411 states that "Federal Reserve Notes . . . shall be redeemed in lawful money on demand" this means only that Federal Reserve banks will exchange the notes on demand for new Federal Reserve notes. Thus today the notes are backed only by the "full faith and credit of the
U.S. government"—the government's ability to levy
taxes to pay its
debts. In another sense, because the notes are legal tender, they are "backed" by all the goods and services in the economy; they have
value because the public accepts them in exchange for valued goods and services. Intrinsically they are worth the value of their ink and paper components."
Source:
Wikipedia.