A seasonal curve is only as meaningful as its contract mapping. A December–May corn spread must represent the same delivery-year relationship in each historical observation.
Write down both legs
Record the product, the near month, the deferred month and the contract year for each. Define the quote as near minus deferred (or the reverse), then keep that sign convention through the chart and P&L. “December versus May” is incomplete until the two years are explicit: December of one year normally pairs with May of the next.
Map each historical season
For every season, use the same relative year offsets. If the current trade studies December 2026 minus May 2027, the comparable prior example is December 2025 minus May 2026. An accidental December 2025–May 2025 pairing is a different trade even if a data query returns prices for both symbols.
Handle incomplete history
Calculate the spread only when both dated legs have valid prices on the same day. A missing quote is not zero. Each hypothetical entry and exit requires prices for both legs. The selected lookback may say 15 years while fewer than 15 completed trades meet those requirements; report the actual count.
Compare like with like
Keep the date window, structure and calculation fixed across years before drawing a conclusion. A constant-maturity series that rolls its legs answers a different question than one named pair of dated delivery months. If you use rolling or stitching, state that method separately instead of mixing it into a dated-contract backtest.
Research the exact spread
Compare dated legs, historical years and current market conditions before using an average curve.
Open Futures Spread Analyzer →Frequently asked questions
Why does the contract year matter?
Cross-year pairs can accidentally become a different structure if the deferred leg is assigned to the wrong year.
Can I fill missing prices with the prior close?
Only if a clearly documented methodology permits it. For a clean same-day comparison, exclude incomplete observations.
Exchange references
Confirm current specifications, listed combinations and margins with the exchange and broker.
Continue the Calendar Spread Trading series
Also see Calendar Spread Trading, Intercommodity Spreads and Butterfly Spreads.
Test the structure yourself
Inspect individual years, trading windows and the full basket before making a decision.
Explore the Analyzer →Educational material only, not investment advice. Illustrative examples exclude trading costs unless stated. Historical results do not guarantee future performance.