Market breadth measures how many stocks are participating in a move, rather than just where the index closed. A broad rally, where most stocks rise, is healthier than a narrow one carried by a handful of giants - even if both produce the same index gain.

How it is measured

Common breadth gauges include the advance-decline line (advancing stocks minus declining ones), the percentage of stocks above their moving averages, and new highs versus new lows. Each answers the same question from a different angle: is the move supported by the whole market, or a thin slice of it?

Why divergence matters

Breadth is most useful when it diverges from the index. If the S&P grinds to new highs but breadth is deteriorating - fewer stocks participating, more making new lows - the rally is running on fumes, propped up by a few heavyweights. That kind of narrow strength is fragile and prone to sharp reversals when the leaders wobble.

A rising index with falling breadth is a parade with fewer and fewer people marching.

Context, not a trigger

Breadth will not time your entries - it is a health check on the broader tape, best read alongside the volatility regime and structure. For an index trader on SPY, weak breadth is a reason for caution and tighter risk, not a standalone signal. It tells you how much conviction is really behind the number on the screen.