City twinning begins for many reasons. In general, twin cities have similar demographics and sizes. They can be created from business links, travel, similar industries, diaspora communities or common history. This is how the partnership between Portland, Oregon and Bologna, Italy, was born of common industries in the field of biotechnology and education and a “similar attitude towards food”, while Chicago`s link with Warsaw, Poland, with the historic Polish community of Chicago began.  The twinning between Indianapolis and Monza, Italy, is due to the long connection between the two cities and motor racing. The term “city diplomacy” is formally used in the work of city and local governments and the C40 Cities Climate Leadership group, and recognized by the USC Center on Public Diplomacy. During a debate in the British House of Lords in March 2014, the development of twinnings towards urban diplomacy was recognised, not only in trade and tourism, but also in culture and reconciliation after conflicts.  The importance of developing “their own foreign economic policy in the areas of trade, foreign investment, tourism and the attraction of foreign talent” was also highlighted by the World Economic Forum. In addition to the C40, the World Cities Summit, the Smart City Expo World Congress are also the Strong City Network and 100 Resilient Cities.   In 2016, more than 125 multilateral networks and forums were deployed to facilitate international cooperation between the various local authorities.  Studies have shown that city mayors play an active role in urban diplomacy. A recent study concludes that geographic distance has very little or no influence on the choice of a partner city.  Twin cities are often chosen for similarities; For example, some fifteen cities are in Wales with cities in Brittany and Oxford with Bonn, Leiden, Grenoble and other university towns.
 In Italy, Rovigo is a good example of pairing with Viernheim, Bedford and Tulcea. Many former cities of West Germany are with former East German cities on the city; these twinnings of cities were established before the fall of the Iron Curtain. The partnerships between Hanover and Leipzig, both of which have large exhibition sites, or between Hamburg and Dresden, are famous examples. The first U.S.-German twinning was found in 1947 between Worthington, Minnesota and Crailsheim.  St. Petersburg in Russia holds the record for the most partnership agreements with other communities.  In June 2012, the Scottish village of Dull and the American town of Boring, Oregon, agreed to give up their communities to promote tourism in both places and played to their names.    In November 2020, U.S. Senator Marsha Blackburn introduced legislation, the Sister City Transparency Act, to provide oversight of the federal government to reduce the risks associated with partner city contracts used for political influence campaigns.  Although the term is often used in a synonymous way, with the term “friendship city,” this may mean a relationship with a more limited scope in relation to a twin city relationship, and friendship-city-relationships are mayor-mayor agreements.  Sister cities or twin cities are a form of legal or social agreement between cities, cities, counties, oblasts, prefectures, provinces, regions, states and even countries located in geographically and politically different areas 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.   The first known twinning in Europe was 836 between Paderborn and Le Mans, France.   From 1905, Keighley, in West Yorkshire, England, had a twinning with the com