What Bollinger Bands measures
The middle curve is a moving average. The upper and lower curves add and subtract a multiple of the lookback window's standard deviation, so the channel widens as price dispersion grows and contracts as dispersion falls.
The bands are relative boundaries, not fixed definitions of expensive or cheap. A strong trend can remain near one band for many bars.
How to read Bollinger Bands
A contraction, often called a squeeze, describes a period of lower recent dispersion. Expansion describes increasing dispersion. Neither state identifies the direction of a later move by itself.
Watch the middle line's slope and how price behaves after touching a band. Repeated movement along an outer band differs from a brief excursion followed by a return through the middle.
AptiStockX default settings
AptiStockX uses a 20-period middle average with upper and lower bands set two standard deviations away, producing three curves with the desktop colors and line styles.
The interactive example starts with only Bollinger Bands enabled. It intentionally uses AptiStockX's synthetic SPY demo data, so the page teaches the charting workflow without presenting live market information.
A practical AptiStockX workflow
Zoom out until several volatility cycles are visible. Compare the width of the channel before, during, and after large price moves, then inspect whether the middle average was rising, falling, or flat.
Use the inspector values to distinguish visual compression caused by chart scaling from an actual reduction in the calculated band distance.
Limitations and interpretation
Band touches are common and do not automatically imply reversal. Standard deviation also reacts to abrupt gaps, so one event can widen the channel for the length of the lookback window.
Interpret the study with trend structure and volume. It summarizes recent price distribution but cannot explain news, liquidity, or future volatility.
Technical indicators summarize historical data. They do not predict outcomes, provide personalized investment advice, or replace independent research and risk management.
AptiStockX is a native desktop stock charting and analysis application for self-directed traders and researchers who want local data, native speed, and a durable workflow.