Well overall this currency issue tends to be the 'basic cause' of price discrepancy between the same equity on the TSX and NYSE. But I have always noticed there is more to it than that but have never figured out what exactly. Look at your chart from March/07 to the extreme outperformance high by June/08 and see the dramatic outperformance of the NYSE:CNQ to the TSX:CNQ.to. Now look at your lower CD chart and see the 1st half of this period CD was rising sharply. And the 2nd half of this time frame CD was trading in a downward sideways trend but the results were for the NYSE:CNQ to still dramatically outperform the TSX:CNQ.to in both time frames. This suggests that while the currency is the basic cause much of the time there is more to it than that,which causes the price discrepancy between them that isn't just intraday traders doing arbitrage etc. There seems to be other factors causing this that can last for weeks if not months at a time other than currency. Bottom line is regardless of what CD did every rally saw the NYSE:CNQ outperform the TSX:CNQ.to. I realize there is more volume and world wide recognition with NYSE listed stocks so perhaps that's all it is. But it is something that is evident often with many stocks I've tracked. Not 100% sure of all the likely causes. Thanks for the chart.