Market internals are real-time gauges of what the broad market is doing beneath the index price. Two of the most-watched by day traders are the $TICK and the TRIN — both measures of breadth and pressure (market breadth).

The $TICK

The $TICK counts how many NYSE stocks are currently trading on an up-tick minus those on a down-tick — a real-time snapshot of momentum across the whole market. Big positive readings (+1000) show broad buying pressure; big negative (−1000) show broad selling (order-flow imbalance). Extremes can mark short-term exhaustion, and a rising index on a weakening TICK is a divergence worth noting.

The TRIN (Arms Index)

The TRIN (or Arms Index) compares the ratio of advancing/declining stocks to advancing/declining volume. Roughly: a TRIN below 1 signals volume flowing into advancers (bullish breadth); above 1 signals volume into decliners (bearish). Extreme readings can flag capitulation or euphoria (fear and greed).

The index is the headline; internals are the byline. A rally the internals don't confirm is a rally on thin ice.

Using internals

Internals are best as confirmation and divergence tools, not standalone signals: they tell you whether a move has broad participation or is narrow and suspect (confluence). Combine them with price structure and dealer positioning for a fuller read of the tape (reading dealer positioning).