San Diego (/ˌsæn diˈeɪɡoʊ/ SAN dee-AY-goh, Spanish: [san ˈdjeɣo]; Spanish for 'Saint Didacus') is a city on the Pacific Ocean coast of Southern California located immediately adjacent to the Mexico–United States border. With a 2020 population of 1,386,932, it is the eighth most populous city in the United States and the seat of San Diego County, the fifth most populous county in the United States, with 3,338,330 estimated residents as of 2019. The city is known for its mild year-round climate, natural deep-water harbor, extensive beaches and parks, long association with the United States Navy, and recent emergence as a healthcare and biotechnology development center. San Diego is the second largest city in the state of California, after Los Angeles.
Xi'an, known as Chang'an and Jiajing in ancient times, is the capital of Shaanxi Province, the vice-provincial city, the core city of Guanzhong Plain urban agglomeration, the starting point of the Silk Road, the core area of "Belt and Road Initiative" and an important central city in western China. It is an important national scientific research, education and industrial base. The "historic city of the world" identified by the United Nations Educational, Scientific and Cultural Organization in 1981. Located in the middle of Guanzhong Plain, it is bordered by the Weihe River in the north, Qinling Mountains in the south and Chang'an in Bashui. Under its jurisdiction, 11 districts and 2 counties are in charge of Xixian New District, with a total area of 10752 square kilometers, with a resident population of 10.0037 million at the end of 2018 and a urbanization rate of 74.01%. The annual gross domestic product of the region is 834.986 billion yuan. Xi'an is one of the important birthplaces of Chinese civilization and the Chinese nation. In history, more than ten dynasties have established their capitals here.