Since 1999, the Schengen acquis (rule of law) has been integrated into the legal framework of the European Union. The essential provisions of the Schengen border code established by Regulation 562/2006 are the removal of internal borders between Schengen countries and the parallel reinforcement of the external borders of the Schengen area. The removal of internal borders guarantees the free movement of EU citizens and qualified third-country nationals. Third-country nationals are subject to extensive monitoring at the entrance and exit of the Schengen area, while EU citizens and others who play the right to free movement are subject to minimum screening for identity purposes. One of the key features of the Schengen area is the Schengen Information System (SIS), a large database used by the national authorities responsible for maintaining public security in the Schengen area and effectively managing external borders. Participating countries are primarily responsible for the management of their external borders, while being allowed to enter into bilateral agreements with neighbouring countries for the implementation of a local border regime. In addition, Schengen countries retain the right to re-establish internal controls for a period of six months in exceptional cases. In December 1996, two non-EU states, Norway and Iceland, signed an association agreement with the countries that signed the Schengen accession agreement. Although this agreement never entered into force, the two countries were part of the Schengen area following similar agreements with the EU.  The Schengen Agreement itself was not signed by non-EU states.  In 2009, Switzerland officially concluded its accession to the Schengen area by adopting an association agreement by referendum in 2005.
 The Schengen area, considered the greatest achievement of European integration, is an area composed of 26 European countries without internal borders; the free movement of EU citizens, their families and qualified nationals; and a common external border.  This is the result of the intergovernmental cooperation of the original signatories to the Schengen Agreement signed on 14 June 1985 by the three Benelux countries, Belgium, the Netherlands and Luxembourg, as well as France and West Germany.  As expected, the main objective of the Schengen Agreement was to phase out border controls between them, while creating safer external borders. An agreement to implement the Schengen Agreement was signed on 19 June 1990.  It has set out detailed rules for the removal of internal border controls, equivalent measures to strengthen external borders and uniform visa procedures. One of the main features of the Schengen area is the Schengen Information System (SIS), used by customs, police, judicial and national border guards to obtain and exchange information on missing or wanted persons, stolen vehicles or documents.   Toby Vogel, Disputed Borders, European Voice.com (June 14, 2012), www.europeanvoice.com/ article/imported/disputed-borders/74600.aspx.