brent is waterborne North Sea crude priced at the coast; cl settles as light-sweet-crude delivered inland at cushing, Oklahoma. The spread is the cost of getting a landlocked barrel to a seaborne market, so it widens when US pipelines or export terminals are constrained and narrows when they are not.
Historically the spread was near zero. The US shale boom stranded barrels in the midcontinent and pushed Brent as much as $25 over WTI in 2011-2012 before new pipelines to the Gulf Coast pulled it back toward the cost of freight.
Traders use it as a pure infrastructure and logistics trade, and refiners use it to decide whether to buy domestic or imported crude.
Example: Brent $82.20, WTI $78.60, spread $3.60. If it costs about $3.50 to move a barrel from Cushing to a Gulf Coast export dock and load it, the spread is at fair value and there is no arbitrage left.
Related: brent, cl, cushing, light-sweet-crude, intercommodity-spread