In the case of a commodity backed currency the guarantee rests upon the value of another commodity, it won't ever be less valuable then a set quantity of said commodity, but obviously the underlying commodity could still collapse.
Which is why
a BASKET of commodities has always been the "gold" standard in measuring and determining constant "value".
And as long as you can exchange your fiat dollars for goods denominated in dollars, they are backed up by the whole US economy and part of the world's...