A differential is the difference between specified prices. Its sign depends on the order of subtraction. If a hypothetical WTI quote is US$80 per barrel and a matching hypothetical WCS quote is US$65, WTI minus WCS is positive US$15; WCS minus WTI is negative US$15. Both describe the same example. These numbers are invented for arithmetic, not market observations.
Before drawing a conclusion, check the benchmark location, quality, date and price type. WCS at Hardisty and WTI at Cushing are different crudes in different places. Their comparison reflects quality and access to refining markets. Pipeline or refinery interruptions can alter the comparison. CER benchmark primer, May 16, 2018.
A wider spread alone does not identify the cause. To investigate a headline, look for dated evidence of a change in transport availability, refinery demand or crude supply. Treat a forecast spread as an assumption. A projection holding the differential constant does not establish that the actual market will stay constant.
The benchmark spread also differs from a company netback. A classroom calculation can show price less chosen costs, but it cannot establish a producer's reported result without the company's definitions and underlying records. Any calculator should show its assumptions and use the same period and currency throughout.
For an editorial graphic, print the subtraction order beside the number. Place an example label above hypothetical inputs, and avoid attaching a live pulse or current timestamp to a scenario. That makes the explanation useful even as the market changes.
This is an educational explainer, not a report of today's differential. Link to dated source observations when applying the method to a specific news event.