When Delta was taking over NW, us NW fliers were told to create a Delta FF account if we didn't already have one. I didn't have a Delta FF # and I was one of the holdouts that refused to create one. At the time I despised the merger lol. I was flying the day they shut down the NW website. I landed in DTW and couldn't check my account anymore. I had one of the 9-digit NW #'s. Delta added a 6 in front of my NW # and changed my last # from a 1 to a 6. Delta slowly won me over and I've generally been very happy with them over the years.