What a quote actually costs

Every resting limit order is an option you have written and given away for free. Post a bid at 100.85 and you have granted the entire market the right — not the obligation — to sell you something at 100.85, for as long as the order sits there. Whoever exercises it will do so precisely when it suits them and not you.

That single observation explains most of what bid-ask spreads do, and it is worth taking slowly.

The option nobody prices explicitly

Options are priced off volatility and time. So is a quote.

Volatility raises the value of the option you have written, because the price is more likely to move through your order before you can pull it. Time works the same way: the longer your order rests, the more chances the market has to find a reason to hit it. A market maker who cannot cancel quickly is writing a longer-dated option than one who can, and must charge more for it.

This is why spreads widen into news. Nothing about the asset’s value has necessarily changed at the moment of widening — but the cost of standing still has.

Two kinds of counterparty

The uncomfortable part is that the option is not exercised at random.

Consider two people who might hit your bid. The first needs cash today and does not much care about the next tick. The second has worked out that the price is about to fall. Both trades look identical on the tape. Only one of them costs you money, and you find out which afterwards.

Market makers call the second kind adverse selection, which is a polite name for being systematically on the wrong side. The spread is what gets charged to everybody in order to survive the subset who know more than you do. The uninformed seller subsidises the informed one. That is not a flaw in the design; it is the design.

A spread is not a fee for the service of trading. It is the premium on an option you did not choose to write, priced for the counterparty you hope you do not meet.

Why the top of book lies

A displayed size of 5,000 at the touch does not mean 5,000 is available. It means 5,000 was available at the moment the message left the exchange, from participants who can cancel in microseconds and will, the instant your intention becomes visible.

The gap between displayed and accessible liquidity is not deception. It is the direct consequence of the option: a quote that cannot be pulled is a quote that must be much wider, so a market of fast, cancellable, tight quotes is exactly what you should expect a healthy venue to produce. The liquidity is real. It is just contingent on you not being the person it is afraid of.

What to take from it

Three things follow, and they are the sort of thing worth checking against your own experience of a market:

1. Spreads are a volatility instrument. When they widen, ask what changed about uncertainty, not what changed about value.
2. Depth is a snapshot, not a promise. Size that vanishes as you reach for it was never mispriced — it was priced against a different counterparty.
3. Passive is not free. A resting order earns the spread and pays for it in adverse selection. Whether that trade is good depends entirely on who is trading against you.

Next week: what happens to all of this when the same instrument trades in fourteen places at once.