Twin or twin cities are a form of legal or social convention between cities, cities, counties, oblasts, prefectures, provinces, regions, states and even countries located in geographically and politically different areas, in order to promote cultural and commercial ties.  The modern concept of twinning, conceived after world War II in 1947, was designed to promote friendship and understanding between different cultures and between former enemies as an act of peace and reconciliation and to promote trade and tourism.  In the 2000s, twinning of cities was increasingly used to establish strategic international trade relations between member cities.   Thus, in the United Kingdom, Liverpool entered into a friendship agreement with Kunming in China. On the other hand, there are many other twin-city partnerships with cities in the UK. In general, friendly cities are less formal than twin cities. A friendship contract is often used as a first step in the urban relationship. If it strengthens and both parties later decide to want a long-term relationship, they will become twin cities. The first known twinning in Europe was between Paderborn, Germany, and Le Mans, France, in 836.   From 1905, Keighley, in West Yorkshire, England, had a twinning with the French communes of Suresnes and Puteaux.   The first modern twinning recorded between Keighley and Poix-du-Nord in the North, France, in 1920, after the end of the First World War.     It was originally the adoption of the French city; Formal partnership charters were not exchanged until 1986.
 A partner city is a long-term partnership between two cities, cities, provinces or, in some cases, countries around the world. The relationship is officially recognized after the most elected official of both parties signed a Memorandum of Understanding that signifies the commitment of both cities to a better future. Alternatively, they will also sign a less committed friendship agreement and rather purely symbolic and social. In the United Kingdom, the term « twin cities » is the most widely used; the term « sister cities » is generally used for agreements with cities in North and South America.   On the European continent, the most commonly used terms are `twin cities`, `twin cities`, `twin cities` and `friendship cities`. The European Commission uses the term `twin cities` and refers to the process as `twinning cities`.   Spain uses the term « ciudades hermanadas, » which means « sister cities. » Germany, Poland and the Czech Republic use the partner city (De) / miasto partnerskie (Pl) / partnerské mesto (Cz), which means « partner city » or « partner city ». France uses twin city (twinning, twinning city or city) and Italy a gemellaggio (partnership) and comune gemellato (common partner).  In the Netherlands, the terms « Partnerstad » or « Stedenband » (« urban borrowing » when it comes to mutual assistance).