Before reaching an agreement with a neighbouring country, the Schengen State must obtain the authorisation of the European Commission, which must certify that the draft agreement complies with the regulation. The agreement can only be concluded if the neighbouring EEA state and the Swiss on the Schengen side of the border area at least grant reciprocal rights and accept the repatriation of those who have abused the border agreement. The agreement introduced the common Schengen visa regime in 1995, which also led visitors to cross-border freedom when they had a Schengen visa. This is how we have actually created a space that can be considered as a single state in which you can travel without having several visas for each country – a single Schengen visa for the whole area! Recently, the rules of the Schengen agreement with those of the European Union have also been taken up, as most Schengen countries were members of the EU. “There can be absolutely no deviation from the solemn commitments made by Northern Ireland at the end of the first phase of the Brexit negotiations. This does not mean a hard border or agreement that would violate the Good Friday agreement. Of the 27 EU Member States, 22 are participating in the Schengen area. Of the five EU Member States that are not part of the Schengen area, four – Bulgaria, Croatia, Cyprus and Romania – are legally required to join the territory in the future, while the other – Ireland – maintains an opt-out. The four member states of the European Free Trade Association (EFTA), Iceland, Liechtenstein, Norway and Switzerland, are not members of the EU, but have signed under the Schengen Agreement.
Three European micro-states that are not members of the European Union, but which are enclaves or half-enclaves within an EU member state – Monaco, San Marino and Vatican City – are de facto part of the Schengen area. At many external border crossing points, there are special routes for EU, EEA and Swiss citizens (as well as family members) and other routes for all travellers, regardless of nationality.  At some border crossing points at the external border, there is a third route for Schedule II travellers (i.e. third-country/EEA/Swiss nationals who are exempt from the visa requirement).  Although Andorran and San Marines citizens are not EU or EEA citizens, they may use special routes for EU, EEA and Swiss citizens.  British citizens will not be able to use the EU`s trace after Brexit under current rules, unless such a right is introduced into the Brexit deal with the EU. Since June 2017, ten urban transport agreements have come into force. A total of 26 countries form the Schengen area by concluding a reciprocal agreement to allow the free movement of all Schengen citizens, to eliminate internal borders and to strengthen external borders.
22 of these countries are members of the European Union, while the other four are members of EFTA (European Free Trade Association). There have been or have there been projects for Lithuania-Russia, Poland-Belarus, Bulgaria-Serbia and Bulgaria-Northern Macedonia on cross-border transport.  The agreement between Poland and Belarus was due to enter into force in 2012, but was delayed by Belarus without a transposition date (october 2012).  Similarly, there are other regions, such as Madeira, the Azores and Guadeloupe, which are on the European continent but are not yet part of the Schengen agreements.