If you’re a member of an airline frequent flyer program or more than one, here’s a new caution.

Up until recently, most airlines published an award/redemption chart, which shows you the minimum mileage needed for a free ticket for a domestic or international flight. You could use it to calculate how many miles you needed to earn to get that ticket.

But that’s not the case any more. A growing number of airlines have gone to dynamic pricing for mileage redemption. What does that mean?

The charts are now gone, and now you won’t find out how many miles you need until you tell the airline the exact date of the flight and class of service.

Therefore, you have all the more reason not to horde your miles and to redeem them as soon as you can.