At the time of their completion, the original 1 World Trade Center (the North Tower) and 2 World Trade Center (the South Tower), known collectively as the Twin Towers, were the tallest buildings in the world. The other buildings included 3 WTC (the Marriott World Trade Center), 4 WTC, 5 WTC, 6 WTC (which housed United States Customs), and 7 WTC. 8 WTC, 9 WTC, 10 WTC, and 11 WTC came later. All of the original buildings were built between 1973 and 1983, with the latter buildings being built from 1984 to 1989.