What a market is

After this lesson you can explain what a market actually does, and why nobody is in charge of what a stock costs.

Winter, 1637

In Amsterdam and Haarlem, in the winter of 1636–37, people traded tulip bulbs in tavern back rooms, and prices for the rarest bulbs reached the cost of a canal house. Most buyers never intended to plant anything. They were buying paper, promises of future bulbs, and planning to sell it on to someone else at a higher price. In February 1637 an auction in Haarlem failed to find buyers, word spread, and within days the paper was worth close to nothing.

Almost four hundred years later, the part worth looking at is that those tavern rooms were already markets in every sense that matters. Somebody would say what they'd pay, somebody else would say what they'd take, and when the two numbers met a trade happened. The agreed number, the price, was public the moment it printed, and people in the next tavern set their own bids off it. The Chicago wheat pit ran on the same mechanism, and so do the servers that trade Apple shares today, with much better plumbing.

What a market does

A market is a meeting place for buyers and sellers, and what comes out of it is the price. Nobody computes what a share of Apple is worth and posts it somewhere official. At any moment there are people willing to buy at some price and people willing to sell at some slightly higher one, and when one of them crosses the distance a trade prints. The "price of Apple" on your phone is the most recent of those prints, and there's no seal of approval on it from anyone.

The mechanism has a name: an auction. Stock markets run a double auction, which means the buyers are bidding each other up while the sellers, on the other side, are undercutting each other. The auction you know from television, one seller and a room of bidders, is half of that.

From under a tree to a data center

The Amsterdam exchange, born around trading shares of the Dutch East India Company from 1602, is the usual candidate for the first real stock market: the first company whose ownership was split into freely tradable pieces. In New York in 1792, twenty-four brokers who had been dealing under a buttonwood tree on Wall Street signed an agreement to trade with each other on fixed terms, and that agreement grew into the New York Stock Exchange. For nearly two centuries after, a market meant a physical floor with humans shouting at each other.

Nasdaq, launched in 1971, was the first market with no floor at all, just a network of dealers connected by screens. Today essentially everything is electronic. The NYSE's matching engine lives in a data center in New Jersey, and the auction itself is a computer pairing the highest bid with the lowest offer millions of times a day. Underneath, it's doing what the tavern did: bids meet offers, and the print goes out to everybody at once.

Why this carries through everything that follows

Two things from this lesson keep coming back.

The first is that a price is the latest agreement between a buyer and a seller, so it moves only when the balance between those two shifts. That's the whole subject of the next course.

The second is that a market with millions of participants is genuinely hard to out-guess. The current price already reflects what all of them, collectively, are prepared to do about everything they know. Much later, in School IV, we build on something subtler than out-guessing: the biggest participants leave traces when they act, and those traces can be read.

Check yourself

1. Who sets the price of a share of Apple? (Nobody. The price is the most recent trade between a willing buyer and a willing seller.)
2. What was actually being traded in the tulip winter, and why did the collapse take days rather than months? (Mostly paper promising future bulbs. The next buyer was the only thing holding prices up, and when an auction found no buyers there wasn't one. Prices are public, so everyone knew at once.)
3. What changed between the buttonwood tree and today's data centers, and what didn't? (Speed and plumbing changed. The double auction, with trades printing publicly, is unchanged.)

The idea this lesson installs

The price is the last agreement, and nobody is in charge of it.

Next: Course 2 — "What price is." (If the tulip story hooked you, Mike Dash's "Tulipomania" tells it properly.)