GLOSSARY // Market Structure
Market Breadth
Market breadth measures how many stocks are participating in a market move, as opposed to how far the index itself traveled. The core reading is advancers versus decliners: 2,400 NYSE stocks up against 500 down is broad strength; an index gain carried by five mega caps while decliners outnumber advancers is narrow.
The standard gauges are the advance-decline line (a running total of daily advancers minus decliners), the percentage of stocks above their 50-day or 200-day moving averages, and new 52-week highs versus new lows. Because cap-weighted indexes can be dragged higher by a handful of giants, breadth is the check on whether the average stock agrees with the headline number.
Divergences do the signaling. When an index makes a new high while the advance-decline line does not — fewer soldiers following the generals — rallies have a documented habit of narrowing before they break. Breadth thrusts point the other way: days when advancers outnumber decliners 9-to-1 cluster near durable market bottoms.
The S&P 500 closes at a record high, up 0.6%, but only 38% of its members finish green and just 44% of NYSE stocks trade above their 200-day moving average, down from 71% three months earlier. The index says bull market; breadth says the advance has thinned to a few large names — a divergence that has preceded several notable corrections.
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Educational only — not financial advice. Definitions simplified for clarity; markets are messier than definitions.