How Prices Form — The Mechanics of Supply, Order Books, and Volume¶

For / Key Points

For: Those who've started investing but are still buying on gut feeling. Anyone who finds news numbers meaningless. Anyone looking to grasp stock market mechanics from a structural perspective.

Key Points:

Price is a record of consensus — a buyer and a seller agreed at that specific price, nothing more, nothing less
The order book maps unrealized intentions — every pending "I'd buy/sell at this price" declaration, but no deal has been struck yet
Volume measures the weight of consensus — the same +3% move means completely different things depending on whether 1 million or 10,000 shares traded

On a brokerage screen, stock prices tick every second. 3,200 becomes 3,201, drops to 3,198.

What's happening behind those numbers? A seller and a buyer agreed on that price. A stock price is a record of consensus — nothing more, nothing less.

The question this article addresses is simple: what is price the "result" of? Whether you hold this premise shapes how you'll read every indicator in the rest of this series.

The Order Book: Unrealized Intentions Made Visible¶

A trading screen displays an "order book." Sell orders on one side, buy orders on the other. The latest execution price sits in between.

Simulated Order Book (5 levels)

  Sell Orders              Buy Orders
  $150.50  200 shares
  $150.40  150 shares
  $150.30   80 shares
            ← Spread →
                $150.00  300 shares
                $149.90  120 shares
                $149.80  200 shares

The essence of the order book is this: it's a collection of unrealized intentions. In the example above, sellers willing to part at $150.30 or higher face off against buyers willing to pay $150.00 or less. The gap between these prices (here, $0.30) is the spread, and a narrow spread means consensus is close.

Crucially, orders on the book haven't executed yet. They can be cancelled at any time. The book is a snapshot of current intentions, not commitments.

There are also orders that don't appear on the public book. Large institutional orders often route through dark pools — private exchanges — or get sliced into small pieces by algorithms. In the US market, dark pools account for roughly 40% of trading volume. The visible order book is only a fraction of the full picture. This point is explored further in a separate article on information asymmetry.

Market examples in this series

This series primarily uses US market examples, but the fundamental mechanics of price formation are universal across markets. Institutional differences between US and Japanese markets are covered in a dedicated article.

With the order book's structure in mind, the next question follows naturally: what actually moves prices on this book?

Market Orders and Limit Orders — The Mechanics of Price Movement¶

Orders come in two fundamental types:

Market orders: "Execute now, at whatever price is available"
Limit orders: "Only execute at my specified price or better"

Market orders are the primary force that moves prices. When a market buy order comes in, it matches against the cheapest available sell limit orders, consuming them from lowest to highest. If the sell side is thin, a single market order can push the price up several steps. A market sell order eats through buy limits, driving the price down.

However, market orders aren't the only force. Limit order cancellations and modifications also reshape the book. If a large buy limit at $150 suddenly disappears, the book's center of gravity shifts downward — even without a single market order executing.

Limit orders typically serve as "walls." If heavy sell limits are stacked at $150, the price tends to stall there. The depth of the order book determines how easily prices can move. This "ease of movement" is the physical foundation of volatility — a concept covered in the VIX article.

The key takeaway: price is determined by how many limit orders a market order consumes and how the shape of the book changes. No matter how strong a company's earnings are, its stock price won't rise without incoming market buy orders. Conversely, a wave of selling unrelated to fundamentals will push the price down.

Volume — Measuring the Weight of Consensus¶

Total shares executed equals volume. What this number captures is how many participants reached consensus within a given price range.

The same "+3% gain" has entirely different characteristics depending on volume:

	High-Volume Move	Low-Volume Move
Meaning	Many participants agreed on that direction	Few trades drove the price
Persistence	Relatively likely to hold	May reverse the next day
Reliability	"Thick" consensus	"Thin" consensus

Volume is a tool for gauging the reliability of price moves. A related concept is the advance-decline ratio, which measures the ratio of advancing to declining stocks across the entire market, distinguishing whether a rally is supported by broad consensus or merely dragged up by a handful of large caps.

Both volume and breadth measure the "reach" of consensus. This leads to one more fundamental question: does a "correct" stock price even exist?

There Is No "Fair Price"¶

There's a calculation method (DCF analysis) that predicts a company's future earnings and converts them to present value, producing a "theoretical price." But the actual stock price almost never matches that number. The reasons operate on two layers.

First, participants are looking at different things.

People who buy because "this company's stock seems cheap relative to its earnings" (value investors) watch cash flows and asset values
People who think "it's been going up, so it'll keep going up" (momentum traders) watch chart patterns and trend strength
Computers that trade automatically in milliseconds (HFT firms) watch order book microstructure and execution speed
Funds that buy the entire market (index funds) watch only tracking error against their benchmark

There's a school of thought holding that all information is instantly reflected in prices (the Efficient Market Hypothesis). If participants processed the same information rationally, prices should theoretically converge to a "correct" value regardless of what each participant focuses on.

But second — and this is the decisive layer — even when participants look at the same information, their interpretations diverge. Two analysts read the same earnings report: one sees "growth deceleration," the other sees "early signs of margin improvement." The same rate hike is read as "headwind for equities" by one side and "inflation control, long-term positive" by the other. Identical information, different conclusions — shaped by experience, time horizon, and risk tolerance.

This is what it means for the market to be an arena of interpretation and anticipation. Price is the provisional consensus reached by participants watching different information and holding different interpretations at a given moment.

With this understanding, "cheap" and "expensive" lose their absolute meaning. A P/E ratio — a number showing how many years of earnings the stock price represents — of 10 might look "cheap," but if the majority of participants have priced in declining earnings, 10x may be "fair." CAPE — a long-term version of P/E that smooths out economic cycles — of 40 might look "expensive," but if participants are front-running an AI-driven growth wave, that elevated CAPE has structural justification.

Indicators show the current state of participant consensus — not the future's correct answer. When studying metrics that gauge whether stock prices are cheap or expensive (valuation metrics) like P/E and CAPE, this premise prevents the shortcut of "the number is high/low, therefore act immediately."

Takeaways¶

Stock prices are not reflections of corporate value but the outcome of participant consensus
The order book is a collection of unrealized intentions — what's visible is only part of the market
Volume measures the "weight" of consensus and is key to judging the reliability of price moves
"Cheap" and "expensive" are relative concepts that shift with participant composition and time horizon

One additional point belongs here. If price is consensus, then the quality of consensus also varies. Calm-market consensus and panic-driven consensus are not structurally the same. The aggregate of deliberate judgment and the aggregate of fear-driven liquidation should not be treated equally. Articles on VIX and sentiment indicators explore how to measure this "quality of consensus."