San Francisco, officially the City and County of San Francisco, is a cultural, commercial, and financial center of Northern California. San Francisco is the 16th most populous city in the United States, and the fourth most populous in California, with 881,549 residents as of 2019. It covers an area of about 46.89 square miles (121.4 km2), mostly at the north end of the San Francisco Peninsula in the San Francisco Bay Area, making it the second most densely populated large U.S. city, and the fifth most densely populated U.S. county, behind only four of the five New York City boroughs.

Seoul, officially the Seoul Special City, is the capital and largest metropolis of South Korea. Seoul has a population of 9.7 million people, and forms the heart of the Seoul Capital Area with the surrounding Incheon metropolis and Gyeonggi province.