When the news says "the market" rose or fell, it usually means one of four US indexes — and they don't always agree. Here's what each one actually tracks, how it's weighted, and why they can move in different directions on the same day.
| Index | What it tracks | Holdings | Weighting | Launched | Best read on |
|---|---|---|---|---|---|
| S&P 500 | Large US companies | ~500 | Market-cap (free float) | 1957 | The broad US market (~80% of its value) |
| Dow Jones (DJIA) | Blue-chip leaders | 30 | Price-weighted | 1896 | A narrow snapshot of big, established names |
| Nasdaq Composite | Nasdaq-listed companies | ~3,000+ | Market-cap | 1971 | Technology & growth |
| Russell 2000 | Small US companies | ~2,000 | Market-cap | 1984 | Small-caps & the domestic economy |
The one idea that explains most of the difference: weighting
Three of the four are market-cap weighted — each company's pull on the index matches its total market value, so the largest companies dominate. The Dow is the odd one out: it is price-weighted, meaning a stock with a higher share price moves it more, regardless of how big the company actually is. That's why a single high-priced Dow member can swing the index more than a far larger company — and why the Dow and S&P 500 can tell slightly different stories on the same day.
Why they move apart
S&P 500 — the default "market"
With about 500 companies covering roughly 80% of US market value, the S&P 500 is what most professionals mean by "the market." It leans toward whatever the biggest companies are doing — in recent years, a handful of mega-cap technology names have driven much of its move.
Nasdaq Composite — the growth tilt
The Nasdaq Composite holds more than 3,000 Nasdaq-listed companies and is heavily weighted toward technology, so it tends to rise faster in optimistic, low-rate periods and fall harder when rates rise or tech sentiment sours.
Dow Jones — the narrow blue-chip gauge
Just 30 large, established companies, price-weighted and chosen by a committee. It's the oldest and most quoted index, but its narrowness and unusual weighting make it the least representative of the four.
Russell 2000 — the small-cap signal
About 2,000 small companies. Because small-caps are more domestic and more sensitive to interest rates and the economy, the Russell 2000 often diverges sharply from the large-cap S&P 500 — a gap traders watch as a read on risk appetite and the health of Main Street.
So which should you watch?
For a single number, the S&P 500 is the standard. Add the Nasdaq to gauge the technology/growth mood, the Russell 2000 for small-caps and breadth, and the Dow as a familiar headline. We report all four every morning, with the same plain-English "Read" on where each sits.
Common questions
Can I buy an index directly?
No — an index is a measurement, not a product. You can buy index funds or ETFs that hold the same companies in the same proportions. This page is educational and not investment advice.
Why is the Dow so often in headlines if it's the narrowest?
It's the oldest US index (1896) and easy to quote as a single big number, so it has cultural staying power — but most investors treat the S&P 500 as the better gauge.
What about international indexes?
Markets like the FTSE 100, DAX, Nikkei 225 and Hang Seng trade while the US sleeps and are a leading tell for the US open — see our international & overnight page and the per-index guides.