The history of Schengen goes back to the 1980s, when several European states considered free movement of citizens alongside the single market.
In 1985, five countries signed the first agreement:
France
Germany
Belgium
Luxembourg
The Netherlands.
The agreement aimed to create an area without internal borders and end border checks within the area.
The source states that citizens living outside the EU but temporarily residing and working in an EU country under a visa agreement may additionally require a Schengen Visa to travel to other EU Member States.
The source lists 22 EU participating countries:
Austria, Belgium, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Poland, Portugal, Slovakia, Slovenia, Spain and Sweden.
The source states that the UK and Republic of Ireland had not signed the agreement, citing security concerns.
Four non-EU states are listed as signatories:
Iceland, Liechtenstein, Norway and Switzerland.
The source therefore gives a total of 26 participating countries at the time of publication.