The operative deflection word being "are", as opposed to "were" - which would have a different answer, and was the basis of the question I asked, which you completely and deliberately dodged.
Steven,
Let's end the confusion.
FRN are not, and never have been IOU's and are in circulation.
They are the only type of U.S. banknote that is still produced today and they should not be confused with Federal Reserve Bank Notes.
Certificates represented a certain mix and amount of ore.
FRN represent themselves. FRN were never, ever, never represented anything else - they did not represent "gold" or "silver" or anything.
FRBN were removed from circulation by 1945.
United States Notes, Certificates, etc. which look about the same, called the same thing, have the same pictures are are the same size but are not FRN.
Most people are confused about this - since the reason for the similarities is completely purposeful - so to confuse the people into thinking they are the same thing.
But they are not the same thing.
PS: I have, myself, muddled them up too in my abbreviations using FRBN instead of FRN - though essentially the same thing.
http://en.wikipedia.org/wiki/Federal_Reserve_Bank_Note
http://en.wikipedia.org/wiki/Federal_Reserve_Note