How Market Making Works: Inventory, Quotes and Spreads
A market maker provides liquidity by offering prices at which it is willing to buy and sell an instrument, subject to size and conditions. It may earn part of the bid–ask spread but also faces losses from inventory movements, trading with better-informed counterparties and difficulties hedging exposure. A narrow quoted spread does not guarantee low total execution costs for every order size.
Bid, ask and quoted depth
The bid is the price a dealer or book participant is willing to pay; the ask is the price at which it offers to sell. The quoted spread is ask minus bid, using consistent units. The accompanying size describes quantity available at each price, subject to cancellation and venue rules. A spread without depth gives an incomplete picture of liquidity because a large order may execute beyond the best quote.
Worked example: gross spread capture
Suppose a dealer buys 1,000 shares at £10.00 and later sells 1,000 shares at £10.04, without any intervening price movement or fees. Gross spread capture is £0.04 × 1,000 = £40. This simple example ignores that the dealer could have been left long while the stock fell, or might not find a buyer at £10.04. Market makers do not automatically complete matched round trips at the originally displayed quotes.
Inventory risk and quote adjustments
If customers keep selling to a dealer, the dealer can accumulate a large long inventory. It may lower bids and asks to encourage buyers, hedge with related instruments or reduce quoted size. If customers keep buying, the dealer may become short and adjust quotes in the other direction. These changes respond to risk management, not necessarily an informed forecast about the asset’s long-run fundamental value.
Adverse selection
A dealer can lose money when trading against a participant who has faster or more accurate information about the instrument’s value. Ahead of significant earnings or economic announcements, uncertainty may cause quotes to widen or disappear briefly. The additional spread compensates for expected adverse selection and risk rather than being pure profit. Some market makers specialise in particular asset types or venues with different competitive conditions.
Exchange order-book versus dealer markets
In a central limit order book, many participants can place limit orders and compete to supply liquidity. In dealer-driven OTC markets, liquidity providers quote bilateral prices and may use multiple hedging venues. The economic role is similar—facilitating transactions—but disclosure, trading rules and execution quality differ. A platform’s last trade or midpoint should not be confused with a firm quote for any amount.
Market impact and slippage
An order larger than available displayed liquidity may consume several price levels, raising the average purchase price or lowering sale proceeds. This is market impact. Slippage can also arise from quote movement between order submission and execution. When evaluating transaction costs, compare the actual filled prices, market conditions, fees and benchmark chosen, rather than assuming all price difference is dealer markup.
How market makers hedge
A stock-options dealer can hedge some delta exposure with shares, while a bond dealer may use related government bond futures or swaps. Hedging can leave basis, gamma, funding and liquidity risks, especially during large market moves. The presence of a hedge does not mean the dealer has removed all risk. Market-maker business models also differ by exchange rebates, technology, capital requirements and market regulations.
Analytical checklist
Check bid, ask, size, time, venue and instrument when interpreting quotes. Consider spreads alongside volatility and depth, and compare actual trading costs with the decision price. Avoid the claim that market makers control or predict every short-term move; many price changes reflect competition among numerous liquidity suppliers and investors.
Related educational material: market liquidity, quoted spreads and depth.
Dealer quotes, inventory and the spread
The quoted bid–ask spread is compensation that may help a dealer cover inventory, adverse selection, capital and execution costs, not a guaranteed margin earned every time both sides are published.
| Illustrative market-making event | Bid | Ask | Risk or outcome |
|---|---|---|---|
| Dealer publishes a two-sided quote | £99.90 | £100.10 | Displayed spread £0.20 |
| Customer sells one share into bid | Dealer buys at £99.90 | — | Inventory grows by one share |
| Customer later buys from dealer at ask | — | Dealer sells at £100.10 | Gross £0.20 spread if both fill |
| Price drops to £99.50 before resale | — | Executable bid may fall | Inventory loss can exceed spread |
A market maker might hedge exposure, skew quotes to attract the opposite flow, widen spreads or withdraw depth when volatility rises. Displayed price and displayed size must both be considered; a quote for 100 shares does not imply an identical execution price for 100,000 shares. Some venues apply maker/taker fees and best-execution rules. Assess changes in quotes, filled volume and inventory risk before interpreting an apparently narrow spread as abundant usable liquidity.
Frequently asked questions
Do market makers always profit from spreads?
No. Inventory losses, hedging costs and adverse selection can outweigh spread revenue.
Is a tight spread a guarantee of liquidity?
No. Executable size and price impact matter.
Are all stock trades with one market maker?
No. Many venues and order participants can supply liquidity.
Primary sources and further education
Related learning: How Interest Rates and Inflation Affect Financial Markets.