Construction Principles and Basic Support/Resistance
The Bollinger Bands (BOLL) indicator, developed by John Bollinger in the 1980s, is a technical analysis tool that integrates the statistical concept of "standard deviation" with a moving average. This combination constructs a dynamic channel that envelops price action. Unlike indicators such as MACD or KDJ, the core function of BOLL is not to predict explicit direction or momentum, but rather to define the relative highness or lowness of price and its volatility range. It utilizes a middle band (MA) and two outer bands (calculated via standard deviation) to delineate the "normal range" of price movement, offering traders a unique perspective on market volatility and potential reversal points.
A complete BOLL system consists of three lines. The Middle Band (MB) is typically a 20-period Simple Moving Average (SMA), representing the market's medium-term trend and the central value of price. The Upper Band (UP) and the Lower Band (DN) are the essence of the indicator. They are not fixed values; instead, they are calculated by adding and subtracting a specific multiple (usually 2) of the standard deviation from the middle band. Standard deviation is a statistical measure of data dispersion, which, in this context, represents the intensity of price volatility. Consequently, when market volatility is high, the distance between the upper and lower bands will widen (expand). When the market enters a consolidation phase with shrinking volatility, the bands will narrow (contract).
The most fundamental application of BOLL is to serve as a dynamic support and resistance band. These three lines collectively form a price channel. According to statistical principles, approximately 95% of all price action should occur between the 2-standard-deviation upper and lower bands. Therefore, the upper band naturally acts as a dynamic resistance level, while the lower band acts as dynamic support. When the price touches the upper band, it suggests the price is relatively high compared to its medium-term trend (the middle band), and the market may be in a relatively overbought state, facing pressure to pull back toward the middle band. Conversely, when the price touches the lower band, it suggests the price is relatively low, the market is in a relatively oversold state, and there is potential for a rebound toward the middle band. The middle band (20MA) itself serves as the "watershed" between bullish and bearish forces, acting as support in an uptrend and resistance in a downtrend.
Volatility Analysis and Advanced Practical Applications
However, the true power of the BOLL indicator lies in its visual representation of changes in volatility, with the most famous signal being the "Squeeze." When the upper and lower Bollinger Bands move extremely close together, it is like a spring being compressed. This indicates the market is in a period of extremely low volatility, namely a consolidation and accumulation phase. This signal itself does not predict direction, but it strongly implies that a powerful, one-sided move (either up or down) is imminent. Traders should remain on high alert during this time, waiting for a breakout in either direction accompanied by volume. Once this breakout (the "Expansion") occurs, it often marks the beginning of a new trend.
Furthermore, BOLL can also be used to identify potential trend reversals. In an uptrend, if price continuously touches or breaks above the upper band, it shows extreme strength. But if a subsequent price rally fails to touch the upper band again (or fails to touch the lower band in a downtrend), this is often an early sign of trend exhaustion. More advanced applications include looking for "W-Bottoms" and "M-Tops." For example, if price touches the lower band and rebounds, and the subsequent pullback does not break the prior low, with the second low forming higher than the first (both near the lower band), this constitutes a W-Bottom, a potential bullish reversal signal.
Although BOLL is unparalleled in defining volatility ranges, its limitation is that it is a trend-following, lagging indicator. It does not generate directional signals on its own but rather describes the state of the price. In a strong, one-sided trend, the price can touch the upper band (in an uptrend) or the lower band (in a downtrend) multiple times without reversing. This situation can easily mislead traders into counter-trend shorting or bottom-fishing in the middle of a trend. Therefore, a rational analyst will never place a reverse trade merely because the price has touched a band. Instead, they will use it as a supplementary tool to judge "relative highs and lows" and combine it with trend confirmation from MACD or divergence signals from KDJ to formulate a comprehensive trading strategy.
