The World Trade Center was built using prefabricated parts which were bolted or welded together at the site. This process dramatically reduced construction time and costs. Using this process, t-beams and other types of cross beams were created and used in each of the World Trade Center buildings. The Twin Towers' collapse sent debris down onto Six World Trade Center, and gutted the interior of that building; the intact cross beam later found in 6 World Trade Center's debris is believed to have come from the North Tower.