The NIE Number is a tax identification number in Spain, known in Spanish as the “Número de Identificación de Extranjero”. It consists of one initial letter followed by 7 numbers and ends with a letter. The NIE’s assigned before July 2008 start with X , after this date it became a Y . Once finished this numeric series they will use the Z.
The NIE Number is issued by the National Police of Spain and in accordance to Spanish law is required for the purchase or sale of any real estate, vehicle, or boat within Spain. There are many other transactions that require the parties involved to possess an NIE Number. The NIE Number itself is issued on a standard A4 page, with the recipients full name (as written on the passport used during the application process), their place of birth, their nationality, and the number itself, along with a stamp and signature provided by the National Police. The document is often used as a form of identification, and in many cases the original document is required (a copy is not sufficient).
Once issued an NIE number, it will never change, irrelevant of marital status, name changes or residence status. The NIE Number is a national number, and therefore the location of application is irrelevant as the number can be used throughout the whole of Spain.
The Spanish government have linked the NIE Number to various other departments such as the Social Security and residence (e.g. including the NIE Number on the tarjeta de residencia - residence card).