[1] Three months before a person's 16th birthday, HM Revenue and Customs (HMRC) notifies them of their NI number.
[2] In 1993, a one-off mass allocation of NI numbers was made to all children under the age of 16 whose parents were in receipt of Child Benefit.
Since 2009, the last two digits determine the day of the week on which various social security benefits are payable and when unemployed claimants need to attend their Jobcentre to sign on (renew their claims): 00 to 19 for Monday, 20 to 39 for Tuesday, 40 to 59 for Wednesday, 60 to 79 for Thursday and 80 to 99 for Friday.
The B, C and D stagger NI Cards had a shorter period of validity in their final year and ran from June, September and December respectively in 1974 until 6 April 1975.
From 6 April 1975 onwards, a computerised National Insurance Recording System (NIRS) was used to allocate all NICs by tax years.
To trace unknown NI numbers, a general index contained millions of small RF2 index slips, filed in order of surname and listing the name(s), date of birth, and NI number for every person within the National Insurance scheme.
Special prefixes used now or in the past include the letters OO (for Tax Credit claims), CR (for investigations), FY (formerly for Attendance Allowance claims, named after the Fylde social security office where claims were processed), MW (used from 1980 to 1987 for migrant workers), NC (formerly for Stakeholder Pensions), PP (for use by pension schemes as PP999999P), and PY or PZ (both used for tax-only accounts created prior to 2003).
[5] Prior to 1984, when a person was allocated an NI number, a manila notification card was issued to them.
From 1984 until 2011, they received a plastic 'numbercard', of similar proportions to a credit card, with the number raised on the front.
[13] National Insurance numbers issued in the Isle of Man hold the prefix MA.