In-sequence registrations have been issued since 1919 starting at G-EAAA to G-EBZZ and then starting again from G-AAAA. The current sequence reached G-CBAA during 2001. Each aircraft in turn is allocated the next sequential mark within a range of three blocks of registrations (e.g. if the next mark to be issued is G-CBAA then an in-sequence mark is any between G-CBAA and G-CBCZ which has not already been issued). Out-of-sequence registration marks are all others which have not currently been issued.
For further details see Availability of Marks.