There were a couple of other buildings which made up the World Trade Center other than the Twin Towers, and those were the Marriott World Trade Center (3 WTC = 3 World Trade Center), 4 WTC, 5 WTC, 6 WTC, and 7 WTC.
Almost the entire complex was destroyed in a terrorist attack on September 11, 2001, but it was rebuilt many years ago, though not to its formal glory.