What the S&P 500 actually is
published 13:17 · updated never, that’s the point
US500 — prices, shape and the desk →
SOFR 3.87% · 22 Sept 2026 · NY Fed
Five hundred companies sit in the index; a much smaller handful decide where it closes.
The S&P 500 moves the way it does because it is weighted by market capitalisation, so the largest handful of companies swing the index far more than their single vote among five hundred would imply, and a broad-sounding number can end up tracking a narrow slice of the market.