I think the question reflects part of the problem. Consider the coinage act of 1792, specifically section 9.
http://memory.loc.gov/cgi-bin/ampage?collId=llsl&fileName=001/llsl001.db&recNum=369
Coins were struck in three metals and the value of one metal was affixed to another metal saying this coin shall contain this much metal but this coin shall be worth half of that coin and contain a different metal.
Contrast that bimetal approach versus saying we define a dollar as this much metal, a quarter as this much metal, and letting the market determine values instead of saying this much silver is worth half of this much gold when the values of silver and gold fluctuate in the market but the amount of metal in coinage does not yet despite metal fluctuates in value this coin will always be worth half of that coin .