It is the only essential UK-wide adult identity reference.
The ‘number’ itself is made up of both letters and numbers, however, they do not include D, F, I, Q, U or V to (historically) reduce handwriting errors… the combinations BG, GB, NK, KN, NT and ZZ are also excluded.
There is a single letter at the very end of an NI number, but this ceased to have a use in 1975 when stamps stopped being pasted to record ca person’s contributions during a year.
Red and blue plastic cards were issued between 1984 and 2011, they are no longer of any use and we are not required to keep them.
Prior to 1992, the school register was used to allocate numbers, therefore an ex-classmate may have the adjacent number to yours.