 There are three lines that make up the Bollinger Bands. The middle line is usually the 20-day simple moving average, the 20SMA. Below the middle line, there is a lower Bollinger Band, which is the 20SMA minus two standard deviations of the last 20 daily closing prices. Above the middle band, there is the upper Bollinger Band, which is the 20SMA plus two standard deviations. Bollinger Bands can also be drawn based on the weekly data, hourly data, etc. The probability of the closing price of a stock falling outside of the upper or lower Bollinger Band is usually only 5-10%. Many traders will get ready to sell a stock when the price is above the upper Bollinger Band. Conversely, when the price is below the lower Bollinger Band, there is a high probability that it will rebound soon. Of course, stock prices are also affected by fundamental issues such as company performances and macroeconomic events.