STRUCTURE Market Structure · March 2026 · ~4 min
Market breadth: the truth behind advancers and decliners
The index makes a new high — time to be bullish? Not necessarily. The index might be lifted by a handful of megacaps while everything underneath falls. Breadth exposes that makeup — it looks not at price but at participation.
Core measures
- Advance/decline (A/D): advancers minus decliners — broad rise or broad fall;
- AD line: the cumulative A/D curve; divergence from the index is an important warning;
- New highs / new lows: the count making new highs vs new lows — a healthy rally shows plenty of new highs.
Watch the divergence
When the index makes a new high but the AD line flattens or slips, the base of the rally is narrowing — only a few leaders holding it up. This "shrinking-circle" rally is often a late-stage signal. Rallies with good breadth travel further.
The index tells you the score; breadth tells you how many are answering the question.
Further: narrowing breadth often means money concentrating into a few sectors — see the sector rotation map; to quantify strength per instrument, see relative strength comparison.