The 747 is a double deck plane. It can carry people, cargo and many other things. Boeing designed the 747's upper deck to be a first class lounge or just extra seats. They also designed the upper deck so that the aircraft could be changed into a cargo plane easily. This was done by removing seats and putting in a cargo door at the front of the plane. Boeing did this because they thought that supersonic airliners would become much more popular than the 747 and other subsonic airliners. However, they thought that subsonic cargo planes would always be popular. It was thought that the 747 would become unpopular after 400 were sold. However, 1,000 planes had been made by 1993, so it stayed popular. By September 2012, 1,448 aircraft had been built. 81 747-8s were still waiting to be built.