Bollinger bands measure volatility. They consist of a moving average, and two lines plotted at 2 standard deviations from the central moving average line. These lines form the band. The market is quiet when the band is narrow. The market is loud when the band is wide.

Bollinger bands are a trading strategy that can be used to identify changes in the market’s volatility. Developed by John Bollinger, the bands consist of a center line and two outer lines that correlate with a percentage of a given stock's standard deviation. The center line is seen as a moving average, and represents the "normal" level of volatility for a stock. The upper and lower bands represent periods where volatility is higher than normal.