Bollinger Bands are a tool of technical analysis which was invented by John Bollinger in the 1980s. Having evolved from the concept of trading bands, Bollinger Bands are an indicator that allows users to compare volatility and relative price levels over a period time. Basically, this tool provides a relative definition of high and low. By definition prices are high at the upper band and low at the lower band. This definition can aid in rigorous pattern recognition and is useful in comparing price action to the action of indicators to arrive at systematic trading decisions. When the market is calm, the Bollinger Band lines get closer together and when the market was changing Bollinger Band line expand.
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You can read rest of the article at Bollinger Bands.