Tiny little indicator. Class A shares of Google (GOOG) have voting power. Class C shares (GOOGL) don't. So Class A sells for more since they're strictly better than class C, but how much more?
With this indicator, we can see that it's usually about +0.5%. So if it's ever different than that, there might be a small arb opportunity as it rebalances itself. i.e. If it's negative, then class C is temporarily more expensive, and you could short class C and simultaneously buy class A. Alternatively, if it's 2% above, you could do the opposite.