How prices form: bids, asks and the spread

No committee sets the price of anything on an exchange. The number flashing on a quote is just the last price at which two strangers agreed to trade, and it can be a different number one second later. Once you internalize that — price is an event, not a property — a lot of confusing market behavior starts making sense.

The machinery that produces those events is a continuous auction called the order book. It runs all day, it has no auctioneer, and everyone in it plays one of two roles at any moment: offering liquidity at a stated price, or taking it. The gap between the two sides — the spread — is the most underrated number in trading. It is the fee nobody itemizes on your statement.

The standing auction

Picture two stacks of orders. On one side, bids: buy orders, sorted from highest price down. On the other, asks (also called offers): sell orders, sorted from lowest price up. A bid says “I will pay this much or less.” An ask says “I will accept this much or more.” As long as the highest bid is below the lowest ask, nothing happens — everyone waits, the book just sits there humming.

A trade occurs only when someone crosses the gap. Either a buyer decides waiting is worse than paying the ask, or a seller decides waiting is worse than accepting the bid. That impatient party is called the taker; the patient party whose resting order gets hit is the maker. Every single trade, in every market on earth, is one impatient person meeting one patient one.

Best bid, best ask and the mid

The top of each stack matters most. The best bid is the highest price anyone will currently pay; the best ask is the lowest price anyone will currently accept. Suppose a stock shows a best bid of $99.95 and a best ask of $100.05. The midpoint, $100.00, is a decent estimate of “fair value right now” — but you cannot trade at the mid. Buyers in a hurry pay $100.05; sellers in a hurry receive $99.95.

Note what this means for the last price you see on a quote: it bounces between bid and ask even when nothing fundamental changes, simply depending on whether the most recent taker was a buyer or a seller. Beginners often mistake this bid-ask bounce for meaningful movement. It is not. It is the sound of the auction breathing.

Why the spread exists

The spread is not a glitch or a rip-off; it is compensation. The people quoting both sides of the book — market makers, professional or amateur — take real risks by standing there, and the spread is what pays them for it:

• Adverse selection. The person trading against a resting order often knows something. If news just broke, the maker’s stale quote gets picked off before they can move it.
• Inventory risk. A maker who buys at the bid now holds the asset and is exposed until they can sell it. Prices can move against them in the meantime.
• Operating cost. Quoting continuously requires capital, technology and attention, none of which are free.

This is why spreads widen exactly when you least want them to. During calm hours in a liquid market, competition between makers squeezes the spread to a sliver. During a news shock, makers either widen their quotes dramatically or pull them entirely — the risk of being picked off just went up, so the price of immediacy goes up with it.

Who pays the spread

Takers pay it, every time, whether they notice or not. Run the round trip: you buy at the $100.05 ask and immediately sell at the $99.95 bid. You did nothing wrong, the market did not move, and you are down 10 cents per share — 0.1% — plus trading fees. That dime went to the makers who quoted both sides.

A 0.1% spread sounds like nothing until you multiply it by frequency. A trader doing one round trip per day at that spread donates roughly 20% of their capital’s value per year to the spread alone, before fees, before being wrong about anything. Frequent trading does not just risk losses — it guarantees costs.

How prices actually move

Prices move in two ways. First, by eating the book: a large buy order consumes everything at $100.05, then $100.10, then $100.15, and the new best ask is higher — the act of buying moved the price. Second, by quote updates: news arrives, and makers reprice their orders before anyone trades at all. A price can gap from $100 to $97 on zero volume simply because everyone simultaneously decided to stand somewhere else. The market does not owe you a fill at any particular number.

Watching it live

Reading about an order book is like reading about swimming. Obsidiate’s trade terminal shows the live book and the trades feed side by side for every instrument; spend ten minutes watching a liquid market and you will see the whole choreography — bids stacking up, asks getting eaten, the spread tightening and relaxing. It is genuinely better than any diagram, and unlike a diagram, it occasionally humbles you.

Key takeaways

• Price is the last matched trade, not an official value; it changes whenever an impatient trader crosses the spread.
• The best bid is the most anyone will pay; the best ask is the least anyone will accept; you trade at those, never at the mid.
• The spread compensates liquidity providers for adverse selection and inventory risk — it widens precisely when markets get scary.
• Takers pay the spread on every trade; at high frequency it becomes a guaranteed, compounding cost.
• Prices move both by orders eating the book and by makers repricing quotes — sometimes with no trades at all.