Richard Arms's index divides advancing issues over declining issues by advancing volume over declining volume. A reading of 1 means volume is distributed in proportion to the number of stocks moving each way.
Counterintuitively, TRIN moves inversely to the market: readings well above 1 mean volume is concentrated in decliners, which is bearish in the moment but is often read as panic and therefore as a possible short-term low. Very low readings suggest the opposite.
It is volatile and its useful thresholds have drifted over time as market structure changed. Like all breadth measures it describes participation, not direction, and single-day extremes are best interpreted alongside price rather than alone.
Related: nyse-tick, market-breadth, advance-decline-line, capitulation, volume-climax