What pairs trading is
Pairs trading means opening opposite positions in related instruments at the same time. You buy the instrument that looks cheap relative to the other and sell the one that looks expensive. The trade is about the spread between them, not the direction of the market. Here, spread means the price gap or ratio between the instruments, not the bid-ask spread.
The idea is that if both instruments move together over time, a temporary gap should close. When it does, the trade is closed. Pairs trading is a type of mean reversion trade applied to a relationship instead of a single price.
In correlation trading forex, typical candidates are currency pairs that share a base or quote currency, such as EURUSD and GBPUSD. Other examples are related metals such as XAUUSD and silver, or stock indices from the same region. See currency pairs for how pairs are quoted.
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Correlation is not cointegration
Correlation measures how closely the returns of instruments move in the same direction. It can range from moving perfectly together, through no relationship, to moving perfectly opposite. It says nothing about whether the price gap stays stable.
Cointegration is a stronger property. Instruments are cointegrated when a weighted combination of their prices tends to return to a stable average, even if each price wanders on its own. That stable combination is what a pairs trader actually needs.
Highly correlated instruments can still drift apart for months. A forex correlation strategy built only on correlation can therefore keep losing while the correlation figure still looks high. Traders who test for cointegration use statistical tests on historical data. These tests describe the past only.
How the spread is built
- Choose the instruments. There should be an economic reason for the link, such as a shared currency, a shared economy or a shared commodity driver.
- Estimate the hedge ratio. This is how much of the second instrument you trade against the first. It is usually found by regression on past prices. It is not simply equal lot sizes, because pip values and volatility differ.
- Calculate the spread. This is the price of the first instrument minus the hedge ratio times the price of the second.
- Standardise it. Many traders convert the spread into a z-score. The z-score shows how many standard deviations the spread is from its recent average.
The calculators page helps with pip value and margin for each leg.
Typical rules
These are common rule types. They are not recommendations.
| Rule | Common form |
|---|---|
| Entry | Open when the z-score moves beyond a chosen threshold |
| Exit | Close when the spread returns near its average |
| Stop | Close if the z-score keeps widening past a further threshold |
| Time stop | Close if the spread has not reverted within a set period |
| Re-test | Recheck the relationship on a schedule and stop trading the pair if it fails |
Both legs are opened and closed together. A leg left open on its own becomes an ordinary directional position.
Hypothetical example
This example is hypothetical. It shows the mechanics only and makes no claim about results.
Suppose EURUSD and GBPUSD have tracked each other closely in past data. Then GBPUSD weakens sharply against EURUSD after UK-specific news, and the z-score moves past the entry threshold. The trader buys GBPUSD and sells EURUSD in the hedge-ratio size.
If the gap narrows, both legs are closed. If the news reflects a lasting change in UK policy, the gap may keep widening. Then the stop closes both legs at a loss.
How pairs trading loses money
- Relationship breakdown. The main risk. A central bank decision, a policy change, a crisis or a structural shift can end the relationship. The spread then trends instead of reverting.
- Costs on both legs. You pay the bid-ask spread and any commission on each instrument, on entry and on exit. Small mean-reversion targets can be eaten by costs.
- Swaps. Holding both legs overnight means a swap on each one. These can add up against you. See swap fees.
- Margin and stop-out. Each leg uses margin. In a fast move, the losing leg can trigger a margin call or stop-out before the hedge helps. A stop-out may close only part of the position.
- Gaps and slippage. Over weekends or news, prices can jump past your stop levels.
- Overfitting. A hedge ratio and thresholds tuned to past data may not hold forward. Use backtesting and forward testing on a demo account to see how fragile the rules are.
- False sense of safety. A "hedged" position is not risk-free. Leveraged CFD and forex trading can lose money fast, including on spread trades.
Position sizing and loss limits are covered in risk management. Pairs trading is different from forex hedging of a single exposure.
Pakistan context
On payments, per the SBP, remitting foreign exchange to overseas forex/margin/CFD platforms is not allowed through any payment channel (FERA s.5(1)). Also, per the SECP, offshore platforms have no Pakistani licence or oversight and offer no legal protection or recourse. Read law and regulators before opening any account.
Frequent questions
Is pairs trading market-neutral?
It aims to reduce exposure to the overall market direction. It still carries spread risk, cost risk and breakdown risk.
Is high correlation enough to trade a pair?
No. Correlation describes returns moving together. A pairs trading strategy needs the price gap itself to revert, which is closer to cointegration.
How often should the relationship be checked?
On a regular schedule you set in your trading plan. Stop trading the pair if the tests no longer support it.
Can I practise this without real money?
Yes. A demo account lets you test the mechanics. Demo fills and costs may differ from live conditions.