Bollinger bands (BB) are similar to envelopes. The difference between them is, the bounds of the envelopes are above and under the curve of a fixed moving average, put into percentage by distance, but the bounds of Bollinger bands are constructed on the basis of distances, which are equal to a certain quantity of standard divergences. The value of standard divergence depends on volatility, so bands control their width. It increases when market is not stable and it decreases during stable periods.
Bollinger Bands are a volatility indicator similar to the Keltner channel.Bollinger Bands consist of: an N-period moving average (MA) an upperband at K times an N-period standard deviation above the moving average (MA + Kσ) a lower band at K times an N-period standard deviation below the moving average (MA − Kσ)