A National Insurance number is used to administer state benefits, but has not gained the ubiquity of its US equivalent, and is not considered proof of identity. As it is the only number that is unique to each individual, does not change during the course of the person's lifetime, and is issued to virtually every adult throughout the UK, it is used by Her Majesty's Revenue and Customs (HMRC) to track individuals for income tax purposes. The number is stylised as LL NN NN NN L, for example AA 01 23 44 B.