Sentence:  Investors have always had to trust somebody or something. . . . But they have not always had to make a leap of faith about a nation's <WSD>irredeemable</WSD> paper currency. Up until Aug. 15, 1971 the dollar was exchangeable into gold at the rate of $35 to the ounce. 
Meaning of the word marked with <WSD> tags:  Not able to be cancelled by a payment or converted to another form of currency or financial instrument, especially one considered more secure or reliable.