Standard Deviation & Historical Volatility

Standard deviation measures how widely price disperses around its average, and historical (realised) volatility annualises that into the standard way of quoting how much an asset has actually moved. This article explains both, how they underpin Bollinger Bands and risk sizing, the difference between historical and implied volatility, and the crucial idea that volatility tends to mean-revert and cluster — so low volatility tends to precede high, and vice versa.