Bollinger bands

Bollinger Bands are envelopes which surround the price bars on a chart. Bollinger Bands are plotted two standard deviations away from a simple short-term moving average. This is the primary difference between Bollinger Bands and envelopes. Envelopes are plotted a fixed percentage above and below a moving average. Because standard deviation is a measure of volatility, the Bollinger Bands adjust themselves to the market conditions.