It's the price difference between Coinbase's BTCUSD pair and Binance's BTCUSDT pair. The higher the premium means Coinbase whales accumulate Bitcoin despite the high price. 2020 bull-run was driven by institutional investors and high net-worth individuals in the U.S., and they're using Coinbase. When the Bitcoin price broke 20k, 30k, and 40k, the Coinbase premium was more than $50.