EURUSD + GBPUSD --> EURGBP.
It could stay stationary, but it could easily trend as well.

Ok, looked at GDAXI and FCHI. A couple questions:

- First thing I notice is that they are rather expensive. Are there cheaper etfs following the same curves?
- Their price is different in magnitude, while % change might be cointegrated. Still, which indicator do you propose to decide when the two prices are "in line" and when they are "apart"?
- I guess the general strategy then would be - sell bigger and buy smaller one, close position when they align again to win the initial discrepancy?

Ratio (spread) indicator between BB is very interesting to make pairs trading

You can use all symbols which are correlated and cointegrated between them

sell the strongest symbol and buy the weakest symbol till reach the medium line of BB. This is my main rule

Thank you for the replies. Excuse my ignorance, but what is BB? Could you describe a bit more details on how you propose to use a ratio to determine spread?

BB are the Bollinger Bands.
Later I post a screen on my use