Here are a few milestones that remind us of the long-term history of the gold/silver ratio according to various monetary scholars:

A ratio of 2.5 to 1 in Egypt under Menes (c. 3100 BC).
A ratio of 6 to 1 in Mesopotamia under Hammurabi (c. 1750 BC).
A ratio of 12.5 to 1 in Greece under Alexander the Great (c. 330 BC).
A ratio of 12.5 to 1 in Rome under Augustus (c. 27 BC).
A ratio of 15 to 1 in most of Europe throughout the Middle Ages.
In 1792, Congress under President George Washington fixed the U.S. gold/silver ratio at 15 to 1.

The geophysical availability of these two elements in the earth's crust puts the natural gold/silver ratio at 17.5 to 1 (silver at 0.07 ppm, gold at 0.004 ppm).