The three classic "opening" (commonly known as trumpet) patterns of Bollinger Bands (BOLL) are as follows:

Opening trumpet (expanding upward)
Pattern: After a long sideways movement, the upper band rapidly rises, the lower band accelerates downward, and the channel expands dramatically.
Meaning: Bullish breakout, often seen at the beginning of a sharp rally, is a buy signal.
Closing trumpet (expanding downward)
Pattern: After a short-term surge, the upper band quickly turns downward, the lower band still rises, and the channel forms an inverted trumpet shape.
Meaning: Bearish strength increases, often seen at the beginning of a sharp decline, is a sell/hedging signal.
Tight trumpet (narrowing)
Pattern: After a long decline, the upper and lower bands gradually converge toward the middle band, and the channel continues to narrow.
Meaning: Balance between bulls and bears, low volatility, often a preparatory phase before a trend reversal (especially a bottom reversal).