Co-terminals are two or more airports that are considered the same for routing and/or fare purposes and can be route specific; i.e., SFO and SJC are co-terminals on routings to from IAH, but may not be for routings to another destination. For example, CO will sell as a standard roundtrip fare IAH-SJC/SFO-IAH because SJC and SFO are co-terminals in that market.

And some airlines will allow you to standby for no charge when ticketed out of one airport and standing by from that airport's co-terminal on the same day as ticketed.

Open jaws, unless using co-terminals, are typically point to point and demand full Y fares, unless that segment has a discounted advance purchase one-way fare.