1.   A new analysis of frequent flier programs suggests Delta Air Lines makes it harder than other carriers to redeem mileage.

2.   Now, accumulated mileage can be redeemed on either airline.

3.   The analysis comes as airlines carry heavy loads of paying passengers, making it tougher to redeem mileage.

4.   The airlines call the new links a boon for consumers, who will be able to accumulate and redeem mileage on more than one carrier.

5.   These business-class passengers are joined by frequent fliers who pay economy fares and redeem accumulated mileage for upgrades.

6.   Unlike today, redeeming mileage for free trips or upgrades was easy, too.