Reading a Spread Chart: How Pairs Traders See a Trade
A stock chart shows you a price. A pairs trader barely looks at it. The chart that matters shows the distance between two stocks — and learning to read it is learning to see the trade itself.
Start with two stocks that move together
Take two stocks in the same line of business — call them Stock A and Stock B. Same industry, same customers, same weather. Most days they rise and fall together, because the forces pushing on one are pushing on the other. Neither is “better.” They are simply related, the way two singers in the same choir are related: different voices, same song.
Now plot the spread: on each day, the difference between the two, adjusted for how much of each stock you hold. What you get is a single line that wanders around a central level — its long-run average, or mean. When Stock A has a strong week and Stock B doesn’t, the line stretches upward. When the excitement fades, it drifts home. That wandering line is the spread chart, and it is the pairs trader’s entire world.
Standardize it, so “stretched” has a meaning
A raw spread is hard to judge. Is today’s gap big, or just Tuesday? The fix is to measure the gap in units of its own normal wiggle — standard deviations, written σ. Statisticians call the result a z-score:
- z near 0 — the pair is at its normal distance. Nothing to do.
- z near ±1σ — a mild lean. Still ordinary weather.
- z near ±2.5σ — a genuine dislocation. The relationship is stretched well past its usual range.
- z near ±3.25σ — a deep dislocation, the kind that shows up rarely.
Drawn on the chart, those levels become horizontal bands. The spread line spends most of its life inside the inner bands, and only occasionally visits the outer ones. Those rare visits are where the trades are.
Where the entry comes from
The rule used here is deliberately patient: no trade until the spread reaches ±2.5σ. Inside that band, the gap is noise — it might close tomorrow or wander further, and there is no edge in guessing. At ±2.5σ, the stretch itself is the information. One stock has been marked up or down out of proportion to its partner, and history says gaps like that usually narrow.
The position is the opposite of the stretch. If Stock A ran ahead, the trade is long Stock B and short Stock A, in matched size — a bet on the relationship, not on either company and not on the market. If the dislocation deepens to ±3.25σ, the system adds once, at the better price the deeper stretch provides. One add, defined in advance — not a ladder of hope on the way down.
Where the exit comes from — and why it’s early
The exit surprises people: the position closes when the spread recovers to ±1.0σ in absolute value, long before it reaches its mean. Three reasons. First, most of the healing happens in that first stretch of the journey home; the last part is the slowest and least reliable. Second, capital sitting in a nearly-healed pair is capital that can’t take the next genuine dislocation. Third, holding out for the perfect exit is how a statistical trade quietly becomes a story you tell yourself.
And the honest part: sometimes the gap isn’t a dislocation, it’s news. One company’s situation has genuinely changed, the old relationship is broken, and the spread is not coming home. That is what a pre-set hard stop is for. A spread chart can’t tell you why a gap opened — it can only tell you the gap is unusual. The rules respect both halves of that sentence.
How to look at a spread chart, in one pass
- Find the mean — the level the line keeps coming home to.
- Check the bands — where is the line now: ordinary weather, or out at ±2.5σ and beyond?
- Ask how it got there — a sharp jump reads differently from a slow drift, even at the same level.
- Remember what the chart can’t say — it shows the distance, never the reason.
That is the whole visual language of pairs trading: two stocks, one line, a handful of bands, and the discipline to act only at the edges. Everything else — selection, sizing, execution — is built on top of being able to read that one chart honestly.