A couple of years ago, Costa Rica switched to a licence plate using an AAA NNN format, excluding vowels, but allowing Y. The previous version, using all numbers was now into the 900,000ths, and they did not want to go to a 7 digit plate were it to reach 1000000. At the time of the switch, for an additional fee, one could obtain a 'personalized plate', but, I didn't have my car at that time, or I would have chosen ZZZ 999, the highest possible plate number. Although I've never seen it on a vehicle, I suspect it was used by the time I needed a licence plate. Because I didn't want a plate that wo