Under a gold standard, each currency is valued in terms of its gold
equivalent.
That is, we look at the price of gold in terms of a currency.
Thus, under a gold standard, there is a system of fixed exchange rates.
A gold standard is in essence a commodity money standard.
If the supply of gold is relatively fixed, the supply of money is also
relatively constrained by the supply of gold, leading to a long-run
price stability.