A 10,000-share volume bar closes once 10,000 shares have changed hands. Unlike a tick-chart, size matters: one block trade can complete a bar on its own.
This normalises the horizontal axis by activity. The open and close of a session, where volume concentrates, produce many bars; the quiet middle produces few. Indicators computed on volume bars therefore respond to real participation rather than to the clock.
Weaknesses include data dependency (consolidated versus single-exchange volume changes the bars), difficulty aligning them with session-based tools like vwap, and the fact that a large late print can complete a bar at an awkward price. Useful for execution-focused traders, awkward for anything that needs consistent calendar comparison.
Related: tick-chart, range-bars, volume, relative-volume