When someone says 'the market was up today,' they almost always mean an index was up. An index is a measuring stick: a rules-based list of stocks, combined into a single number that tracks how the group is doing.
An index isn't a fund, a company, or anything you can buy directly. It's a calculation, published by firms whose whole business is maintaining these lists. Index funds are separate products built to copy an index's holdings — the index is the recipe, the fund is the dish.
The recipe has two big ingredients: which stocks are in the list, and how much each one counts. That second part — weighting — matters far more than beginners expect. Two indexes holding identical stocks can tell different stories on the same day if they weight them differently. The rest of this chapter is mostly about that one idea, because once you understand weighting, index behavior stops being mysterious.
The S&P 500 tracks roughly 500 large U.S. companies, and it weights them by market capitalization — share price times shares outstanding. Bigger companies count more, in proportion to their size.
A tiny worked example shows the machine. Imagine an index of two companies: Alpha, worth $900 billion, and Beta, worth $100 billion. Total value: $1 trillion. Alpha gets a 90% weight, Beta 10%. If Alpha rises 1% and Beta falls 5%, the index moves 0.9% minus 0.5% — up 0.4%, even though half the stocks fell. Size drove the answer.
That's cap-weighting's signature, at any scale. When the biggest companies grow huge, they dominate the number: in recent years, the top ten names have made up roughly a third of the entire S&P 500. On days when the giants and everyone else disagree, the index reports the giants' day.
Cap-weighting has a genuine elegance — it mirrors the actual market, and it never needs rebalancing as prices move, since weights adjust themselves. But remember what it is: a giant-weighted average, not a democracy of 500 equal votes.
The Dow Jones Industrial Average is the most famous index in the world, and it's built on the strangest math. It holds just 30 large companies, and it weights them by share price alone.
Sit with that. A stock trading at $300 carries ten times the weight of a stock trading at $30 — regardless of which company is actually bigger. The $30 stock could be worth five times more as a company; the Dow doesn't care. A 1% move in the $300 stock moves the index ten times as much as a 1% move in the $30 one. Split a $300 stock four-for-one and its Dow weight quarters overnight, though nothing about the business changed.
Why such a quirky design? History. Charles Dow started the average in 1896 by adding share prices and dividing by the count — arithmetic a person could do by hand. Today a running divisor adjusts for splits and substitutions, but the price-weighted core remains.
The Dow endures as a cultural landmark, but 30 stocks weighted by price is a peculiar lens, and professionals overwhelmingly benchmark against broader cap-weighted indexes.
Modern cap-weighted indexes add one refinement: they count only the float — shares actually available to public investors — rather than every share that exists.
Why? Some shares essentially never trade — founders may hold 40% of a company, governments hold stakes, one company may permanently own a chunk of another. Weighting by total shares pretends there's more investable stock than the market can buy.
A quick example. Two companies are each worth $100 billion total. One's shares are 100% publicly held; the founder of the other still owns half. On a float-adjusted basis, the first counts at $100 billion and the second at $50 billion — half the weight, despite equal total size.
This isn't trivia. Trillions of index-fund dollars buy stocks in proportion to weights, and ignoring float would force funds to chase shares that barely trade, distorting prices. Major providers moved to float adjustment in the early 2000s largely for this reason.
Beyond the two celebrities, a whole family of indexes serves different purposes.
Total-market indexes hold thousands of stocks, adding the small and mid-sized companies the S&P 500 leaves out. The Russell 2000 tracks small U.S. companies — the number people check to see how smaller businesses are faring. The Nasdaq-100 holds the largest non-financial companies listed on the Nasdaq exchange, which in practice has made it technology-heavy. Canada's headline benchmark is the S&P/TSX Composite, dominated by financials, energy, and materials — a very different mix from its U.S. cousins, which is why the two countries' markets can diverge for years.
Equal-weight versions count every constituent the same, and comparing them to the regular index is a neat diagnostic: when cap-weighted wins, giants are leading; when equal-weight wins, the average stock is.
One last mechanic: when a stock joins or leaves a major index, funds must buy or sell it, and that can move prices around the announcement.
Indexes touch your investing life in three practical ways.
First, they're your benchmark. Any strategy you try — a stock you pick, a screen you build — should be measured against what a boring index fund would have delivered over the same stretch. Beating the market sounds thrilling until you define the market; the index is that definition. Professionals fail to beat their benchmarks with humbling regularity — worth remembering before assuming you will.
Second, they're investable through cheap index funds, where weighting tells you what you actually own: a cap-weighted fund is, increasingly, a concentrated bet on its largest names riding on a diversified base.
Third, they're the yardstick for testing ideas against decades of history — a strategy that made money but trailed the index for twenty years is an answer, just not the one its inventor hoped for. Whenever you see a chart of 'the market,' ask: which index, weighted how? The answer changes the story more often than you'd think.
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