They are executed to yield an overall net position whose value, called the spread, depends on the difference between the prices of the legs.
[3] A common use of the calendar spread is to "roll over" an expiring position into the future.
When a futures contract expires, its seller is nominally obliged to physically deliver some quantity of the underlying commodity to the purchaser.
In practice, this is almost never done; it is far more convenient for both buyers and sellers to settle the trade financially rather than arrange for physical delivery.
Intercommodity spreads are formed from two distinct but related commodities, reflecting the economic relationship between them.