The Spread

Let me tell you what the spread actually is, because the textbook definition is almost useless.

The textbook says: the spread is the difference between the price a buyer will pay and the price a seller will accept. That's technically correct. It's also like describing music as "organized sound." True, and completely missing the point.

What the spread is, in practice, is the cost of certainty. When you want to buy something right now, with no waiting, no negotiating, no risk that the price moves before you get there — you pay the offer. The person selling to you is taking on the uncertainty you're offloading. They want to be compensated for that. That's the spread.

In liquid markets — large-cap equities, major currencies, on-the-run Treasuries — the spread is so thin it barely registers. A fraction of a cent. The market is so efficient, so full of competing flow, that certainty is nearly free.

In everything else, it isn't.

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The desk I sit on covers a lot of names that aren't large-cap liquid. Small and mid-cap stuff. Names where a big order can move the price, where the spread tells you something real about supply and demand, and where having an axe on one side or the other actually matters.

When I first started, I thought the spread was just friction — something to minimize, a tax on trading. My first desk head disabused me of that pretty quickly.

"The spread is information," he told me. "It's the market telling you how hard it thinks this trade is going to be."

I've spent the better part of fifteen years thinking about variations on that sentence.

A widening spread usually means one of a few things: the market is uncertain about what the name is worth, there's a known event risk ahead (earnings, data), or someone with better information than the rest of us wants to transact and the market knows it. Sometimes it's all three at once and you don't know which until after.

A tightening spread, particularly in something that had been wide, often means the uncertainty has resolved itself one way or another. Or that so much two-sided flow has come in that the market has found its equilibrium.

Neither of these things is guaranteed. But over time, they're not random either.

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I'll end with something that sounds simple but took me an embarrassingly long time to internalize: in illiquid names, the spread is your first trade. Before you do anything, you've already paid it. That changes the math of what's worth doing.

—C