The Bollinger Bands, on the other hand, also use a preset simple moving average (SMA) as the center of it s three line array. I generally see the Bollinger Band SMA set around 20, but any number will cause a set a bands to be formed and Bollinger, in his book, thought variations on the twenty period SMA in different markets could produce sacrosanct results. Instead of using a preset multiple of the SMA the Bollinger Bands set the outer lines at two standard deviations from the center line. The level of standard deviation can be altered, but the generally accepted norm seems to be about two standard deviations. So we are dealing with a non-linear outer line formation now, since the standard deviation changes in size depending upon the position of the center line. When the market is consolidating, Bollinger Bands tend to draw very close together, showing a very low level volatility. Conversely, when the volatility is increasing, the bands will swing wildly away from each other and the width between the outer bands becomes greater.