The following people are typically eligible for interline rates:
Airline employee or retiree, spouse, dependent child under age 19, parent, parent-in-law with some lines.
Anyone sharing a cabin with one of the above people.
An additional interline cabin for friends is allowed with some cruise lines.
Extended family & friends can often obtain special rates without an eligible interliner.
All interline bookings require proof of eligibility (airline ID and or employment letter) to be submitted at payment time.