Oil is usually quoted as one price. It is at least two.

On Friday, Nov. 6, Brent crude futures added 3 cents to settle at $98.32 a barrel, CNBC reported. U.S. West Texas Intermediate futures gained 31 cents to settle at $90.83. For the week, Brent fell more than 16 percent and WTI lost nearly 13 percent.

Those are two different contracts on two different crudes. Both are benchmarks, and both are futures.

What a futures contract is

A futures contract is a standard contract to buy or sell a specific commodity of standardized quality at a certain date in the future, according to the U.S. Energy Information Administration. A producer can lock in a price by selling a futures contract. A refiner or other consumer can lock one in by buying.

A spot transaction is different. It is the purchase of a single shipment for prompt delivery at the current price.

Because the contracts are standardized, they can trade in volume, and the prices they produce become reference points. More than 1 million WTI futures and options contracts trade daily, according to CME Group, which lists WTI on the New York Mercantile Exchange.

What a settlement is

The settlement is the price the exchange posts for a contract at the end of the trading session. It is the figure reported as the day's price. When a headline says WTI "settled at $90.83," it means that contract's official price for Nov. 6 was $90.83 a barrel.

NYMEX WTI futures are physically settled. A contract still held at expiration is completed by delivering crude oil, not by a cash payment. That delivery obligation ties the futures price to physical barrels.

Two crudes, two regions

West Texas Intermediate is a U.S. light sweet crude blend. North Sea Brent represents the price of light, sweet crude in Europe, CME Group says.

The two benchmarks price different crudes delivered in different regions. Each reflects supply and demand where its oil changes hands, so the two prices rarely match. Depending on conditions in each market, either one can price above the other.

On Friday, Brent settled $7.49 a barrel above WTI.

Why the prices move fast

The EIA describes oil markets as essentially a global auction. Geopolitical events and severe weather that disrupt flows can affect prices. In the short term, both supply and demand are inelastic, meaning neither adjusts quickly to a change in price, and the agency ties that to oil's volatility.

The past week supplied the events. Iran reported three additional attacks on oil tankers in the Strait of Hormuz on Thursday, and UKMTO data indicated attacks on vessels continuing at a near-record rate, CNBC reported. Investors weighed the risk of renewed military escalation between the U.S. and Iran.

On Saturday, after Friday's settlement, Iran's Revolutionary Guard said it had lifted a self-imposed 2,000-kilometer (1,200-mile) limit on the range of its ballistic missiles, the AP reported. Futures were not trading when that was announced. Any effect will show in the next settlement.

Alongside the headlines, the market watches physical supply data. The EIA's weekly inventory report comes out on Wednesdays, according to CME Group.