Bollinger Bands

The bands sit two standard deviations above and below a 20-day moving average, so they widen when volatility rises and squeeze when it falls. Roughly 95% of closes fall inside them by construction.

Two classic reads: the squeeze (unusually narrow bands mark volatility compression that tends to resolve violently) and band walks (in strong trends price rides the upper or lower band for weeks — touching a band is not by itself a reversal signal).