In this way, in the world of ships, the left side of the hull has become the port side, and has been used for getting people on and off since early times. Consequently, in the 20th century with the advent of air travel, people began to board and disembark through the left side of planes just as on ships. Over time, many airport facilities, including the boarding bridge, began to be designed on the basis of left side boarding/disembarking, and the left side of planes became port side as well. With few exceptions, almost all airports now use the left door for passengers to get on and off.