A contract's specification is the part nobody reads until it matters. It states the quantity, the deliverable grade or the index settled against, the delivery location, the listed months, the last trading day, and the delivery mechanism. Each of those has been the cause of a well-known dislocation at some point.
The expiry rule is the one that affects daily work. Each contract root expires on its own calendar rule, not on a shared date. WTI stops trading three business days before the twenty-fifth of the month preceding delivery; Brent expires at the end of the second business day after the last day of the second month preceding delivery; the refined product contracts expire on the last business day of the month preceding delivery. These are different rules, and they produce different answers.
The delivery mechanism divides the benchmarks into two families. WTI is physically deliverable at Cushing, Oklahoma, which is what ties it to a specific set of tanks and pipelines and gives it a genuine local supply-and-demand problem. Brent settles financially against an index of physical North Sea assessments, so its anchor is the assessment process rather than a delivery point. Neither design is better; they have different failure modes, and both failure modes have been realised in living memory.
The deliverable-grade clause is looser than the benchmark's name implies. The WTI contract accepts several domestic streams within stated gravity and sulphur bands, not only West Texas Intermediate itself, and the seller chooses which to deliver. A rational seller delivers the cheapest qualifying barrel, so the contract effectively tracks the least valuable grade that meets the spec. That is a small, permanent, and entirely legitimate drag on the contract relative to the grade it is named after.