Liquidity: What It Is and Why Every Trader Should Care (Chapter 19)
Everyone talks about liquidity. Traders talk about it. Regulators talk about it. Financial journalists definitely talk about it. But Harris makes a sharp observation right at the start of Chapter 19: rarely does anyone define what they actually mean. People use the same word to describe different things, and then they wonder why they cannot agree on anything.
This chapter fixes that. And it is one of the most important in the entire book.
Liquidity Is a Search Problem
Here is Harris’s core definition: liquidity is the ability to trade large size quickly, at low cost, when you want to trade. Simple enough. But notice it has three components. Size. Speed. Cost. Those three things are in constant tension with each other.
Harris frames liquidity as the outcome of a bilateral search. Buyers search for sellers. Sellers search for buyers. When they find each other at mutually acceptable terms, both have found liquidity.
This framing is powerful because it immediately explains why liquidity is so complicated. In a unilateral search, you are just looking for the best deal. You search until the expected benefit of one more inquiry is less than the cost of making it. But in a bilateral search, there is an extra complication: while you are out searching, the matches you already found might disappear. The seller who was willing to trade with you five minutes ago might have already traded with someone else.
Harris uses a wonderful analogy to explain this. Finding liquidity is like finding a dance partner at a school dance. Some people ask others to dance directly (active searchers, demanding liquidity). Some stand on the edge of the floor bouncing to the music, clearly interested but waiting to be asked (displayed limit orders). And some lean against the wall, seemingly uninterested, but they might say yes if the right person asks (latent liquidity, undisclosed orders).
Two wallflowers will never dance with each other. Likewise, two purely passive traders will never find each other. Someone has to make the first move.
The Three Dimensions
Harris breaks liquidity into three measurable dimensions.
Immediacy is how quickly you can trade a given size at a given cost. This is what most retail traders care about. Can I get my order filled right now? Market orders demand immediacy.
Width is the cost of doing a trade of a given size. For small trades, this is basically the bid/ask spread plus commissions. Width is the price of liquidity per unit.
Depth is how much you can trade at a given cost. This is what institutional traders care about. Can I move 500,000 shares without moving the price against me?
These three dimensions are interconnected through tradeoffs. If you are willing to wait longer (sacrifice immediacy), you can usually find more depth or narrower width. If you want to trade more size (demand more depth), you will pay wider spreads or wait longer. If you offer better prices, you will find more depth and wait less time.
Width and depth are actually mathematical duals of each other. The strategy that minimizes cost for a given size is the same strategy that maximizes size for a given cost. They are two sides of the same coin.
There is also a fourth dimension that traders and academics reference: resiliency. This is how quickly prices bounce back to normal after a large uninformed trade pushes them away. A resilient market absorbs big orders without lasting price impact. We will see later that value traders are the primary source of resiliency.
Who Supplies Liquidity
Harris identifies five types of liquidity suppliers, each specializing in a different niche. This framework is incredibly useful for understanding how markets actually function.
Market makers supply immediacy to small traders. They quote bid and ask prices and stand ready to trade at those prices. They trade frequently, try to avoid large inventory positions, and generally do not know much about fundamental values. Their edge is knowing what prices produce balanced order flows. They quote narrow spreads but only for small size. If asked to trade big, they widen their quotes substantially because they fear informed traders.
Market makers need capital to finance their inventories. But raising capital is hard because investors worry about incentive problems. Most people do not work as hard for others as they do for themselves. Market-making firms that do raise outside capital need excellent risk management systems and compensation structures that align trader incentives with investor interests.
Block dealers supply depth to large uninformed traders. They know their clients well and will only facilitate trades for clients they believe are not trading on inside information. Because they need to evaluate why their clients want to trade, they are slower than market makers. But they can offer much larger size because their knowledge of clients solves the adverse selection problem.
Value traders are the ultimate suppliers of liquidity. When nobody else will trade, value traders will trade if the price is right. They know more about fundamental values than anyone else, which lets them take positions that other traders find too scary. They make markets resilient because they step in after uninformed trading pushes prices away from fundamentals.
Value traders are slow. They need to be very confident about values before committing capital. They trade after the price impact has already happened. But their willingness to buy when prices are depressed and sell when prices are elevated is what pulls prices back to fair value.
Precommitted traders are people who already want to trade for other reasons (investing, hedging, rebalancing) but use limit orders to get better prices. They supply immediacy through aggressive limit orders. They can offer narrower spreads than dealers because they do not need to cover the costs of running a dealing business. But they typically do not offer much depth because displaying large limit orders invites front runners.
Arbitrageurs do not really supply liquidity themselves. They are porters of liquidity. They demand liquidity where it is cheap and supply it where it is expensive, effectively connecting fragmented markets. When traders in one market are buying and traders in another market are selling, arbitrageurs link those demands. They increase depth by making liquidity from any related market accessible.
A Practical Example
Harris illustrates all of this with a detailed scenario. An impatient uninformed trader sends a large sell order to the NYSE floor. The specialist is quoting a tight market for small size. The order is much bigger than what is available.
The specialist pages other floor brokers. They gather interest from clients. The price needs to drop from 40 to 37 to fill the order. Limit orders on the book fill first. Floor broker clients take a chunk. The specialist buys the remainder for his own account.
Now the specialist has too much inventory. He lowers his quote to attract buyers. Soon, an options arbitrageur sends a market order to buy. The arbitrageur simultaneously sells calls and buys puts in the options market, creating a synthetic short. The arbitrageur demands immediacy from the specialist but supplies depth indirectly, because the options traders on the other side are also participating.
Days or weeks later, a value trader buys from the specialist at 38.25. The value trader makes markets resilient by restoring prices toward their former levels.
The large uninformed trader, meanwhile, lost money by trading too aggressively. His behavior looked like an informed trader’s, which scared everyone into demanding worse prices. Had he been more patient, or had he been able to credibly signal that he was uninformed, he would have gotten much better execution.
Why This Matters
Understanding liquidity is not academic. It has direct practical implications.
If you are a trader, you need to know which dimension of liquidity matters for your situation. Small retail traders mostly care about immediacy and width. Large institutional traders care about depth. The strategy you use to find liquidity should match what you need.
If you are evaluating a market’s quality, you need to consider all dimensions. A market with tight spreads but no depth is not truly liquid for large traders. A market with great depth but no immediacy is useless for traders with urgent needs.
And if you are thinking about market structure, you need to understand the ecosystem of liquidity suppliers. Each type serves a function. Market makers handle the routine flow. Block dealers handle the big trades. Value traders provide the safety net. Precommitted traders narrow the spreads. And arbitrageurs connect everything together.
Take away any one of these players and the market gets worse for everyone.
This post is part of a series on Trading and Exchanges: Market Microstructure for Practitioners by Larry Harris (Oxford University Press, 2003).