What an index measures

An index is a number computed from a list of companies by a published rule. That is all it is, and every word of it matters.

What it actually is

Three parts:

A list. Which companies are in it, chosen by rules about size, liquidity and listing history.

A weighting. How much each one counts. Most modern indices weight by free-float market capitalisation — company size, counting only the shares actually available to trade, which excludes promoter and government holdings that never come to market.

A rule for changes. Companies are added and removed on a schedule when they stop meeting the criteria.

Everything people say about "the market" is a statement about one of these constructed numbers.

The big companies dominate

Because the weighting is by size, an index is not an average of its companies — it is an average weighted heavily towards the largest few.

In a size-weighted index of fifty companies, the top handful can account for a third or more of the whole. When they move together, the index moves with them, and the other forty-odd barely matter that day.

Which produces the most common confusion in market commentary:

The index was up 1% and most of my shares fell.

Both can be true at once, and usually are more often than people expect. The index reports what a weighted basket did. It does not report what the typical share did, and it certainly does not report what your shares did.

What an index leaves out

Companies that failed. Firms that collapse are removed. The index continues with the survivors, so its long-run record reflects a list that was repeatedly cleaned — a real effect worth knowing about when reading any long-term chart.

Dividends, in the headline version. A price index counts price only. Total-return versions count reinvested dividends and are meaningfully higher over long periods. When someone quotes a long-run index return, it is worth knowing which they mean.

Everything not in the list. An index of fifty large companies says nothing about small ones, and an index of one country says nothing about the rest.

Why anyone should care

Two practical uses, and one misuse.

As a benchmark. If a fund charges you a fee to beat the market, the index is the market it should be measured against. Without it, "we returned 14%" is unanswerable.

As a thing to own. An index fund buys the list in the stated weights. The appeal is not brilliance; it is that the rule is public, the cost is low, and no judgement is being paid for.

The misuse is treating the index as a forecast, or as a description of your own portfolio. It is neither. It is a measurement of a specific list under a specific rule.

Reading a claim about "the market"

Three questions that make most market commentary legible:

1. Which index? Large companies and small ones routinely move in different directions.
2. Price or total return? The difference compounds.
3. Over what period, ending when? A start date chosen just after a crash, or just before one, can make almost any argument.

None of these is a sophisticated question, and asking them removes most of the noise.

The point

An index is a rules-based, size-weighted number computed from a chosen list. It is useful as a benchmark and as something to own cheaply. It is not the average share, it is not your portfolio, and it does not include the companies that did not make it.