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Rationale behind using 12, 26 and 9 to calculate MACD
The values of 12, 26 and 9 are the typical industry standard setting used with the MACD, however other values can be substituted depending on your trading style and goals. The 26d EMA is considered the long moving average when in this case it is compared to the shorter 12d EMA. If you used a 5d EMA and a 10d EMA then the 10d EMA would be considered the long MA. It is based on what you are comparing it with. Apart from providing signals for a reversal in trend, MACD can also be used as an early indication to a possible end to a trend. What you look out for is divergence between the price and the MACD. See chart below of an example:  Here I have used 10d & 3d EMAs and 1 for the signal (as I did not want the signal to show up). I am simply using the MACD as a momentum indicator - which work by providing higher highs in the MACD with higher highs in price. This shows that the momentum in the trend is good so the trend should continue. However the last high in price is not met with a higher high in the MACD. The green lines demonstrate bearish divergence between price and the MACD, which is an indication that the momentum of the trend is slowing down. This could provide forewarning that the trend may be about to end and to take caution - i.e. not a good time to be buying this stock or if you already own it you may want to tighten up your stop loss.