With Northwest's exit, Delta has the world's largest mileage program.

In the early morning hours of Oct. 1, Northwest Airlines pulled the plug on its frequent-flier program. After a 23-year run, the WorldPerks program had issued its last frequent-flier mile and taken its place as a footnote in the history of commercial aviation.

It was WorldPerks, in conjunction with MCI, that first offered frequent-flier miles for phone service, in June 1988. Miles-for-dining were first offered by WorldPerks, in March 1993.

Although Northwest wasn't the first to allow members to combine frequent-flier miles and cash, its periodic Cash and Miles promotions helped establish the practice as an industry standard.

NORTHWEST TO DELTA

WorldPerks may be gone, but WorldPerks members' miles live on.

As active members of that program should know, all their miles have already been transferred to Delta, or will be by the end of October. Depending on their situation, WorldPerks members will experience one of the following three consolidation scenarios:

1. WorldPerks members who were not already members of the SkyMiles program will find that instead of a WorldPerks membership number, they now have a SkyMiles number. The number of miles — both redeemable miles and elite-qualifying miles — in their accounts will not have changed.

2. WorldPerks members who also have SkyMiles accounts and already manually linked their accounts should now have a single account, with a SkyMiles membership number, combining the miles from both programs.

3. WorldPerks members who also have SkyMiles accounts and did not manually link their accounts will find their accounts have merged automatically, provided the names and other contact information in the two accounts matched. If the information did not match, the merger will have resulted in a second SkyMiles account. In that case, the member will have to manually consolidate the two accounts.

Even given Delta's best intentions, WorldPerks members should ensure that their miles are properly transferred to Delta.

• Check there is a single account, with a SkyMiles membership number that includes all Delta and Northwest miles and reflects their elite status.

Delta expects to complete the conversion process and notify customers by Oct. 31.

• Review the SkyMiles rules and policies especially appropriate for new members. One major difference: Whereas Northwest miles expired after 36 months, Delta miles disappear if there's no account activity for 24 months.

• Change is rarely painless; one source of discontent is the loss of PerkChoice — the ability to pay for half of a round-trip with cash and use miles to pay for the other half.

But Delta has shown resolve in capitalizing on its new status as the world's largest airline and operator of the largest airline loyalty program, backing up its promise that SkyMiles aspires to be "Best in Class" with enhancements to the program, including rollover elite miles.