First published by Money Metals here: https://www.moneymetals.com/news/2016/06/13/platinum-vs-gold-price-000879

We often cite the gold: silver ratio as an indicator for the precious metals markets. When the ratio is high (gold is relatively strong versus silver), that typically corresponds with a cyclical low area in precious metals prices. When the ratio starts narrowing (silver gains strength versus gold), that typically corresponds with a cyclical bull market in precious metals.