I believe 20 years is correct. I think it was also for the planes on property at point in time. If there were a merger today and Delta ordered more 777, 747, A330's, or some A350's for example (ie new category or category is grown bigger) then the fences would not apply and Alaska guys could bid those aircraft. If I am not mistaken some Republic MD-80 Captains were able to move to 747 Captain in this way.