Bollinger bands

Used in technical analysis to determine areas of support for and resistance to price changes. On a chart these plot the standard deviation of the moving average of a price. So when they are plotted above and below the moving average, the bands widen and narrow according to the underlying volatility of the average. The longer the period of low volatility, the closer together the lines become and the greater is the likelihood that there will be a break-out from the established price pattern.