The term "direct flight" is not legally defined in the United States,[3] but since the 1970s the Official Airline Guides have defined the term simply as a flight(s) with a single flight number.[3] (In earlier years "direct" in the OAG did mean "no plane change".) While so-called "direct" flights may thus involve changes in aircraft (a "change of gauge"), or even airline[3] at the intermediate point, they are typically—but not always—differentiated from "connecting flights" in that the airline will enforce a dependency between multiple legs of the flight, so that leg two cannot operate if leg one has failed to arrive at the departure airport. Direct flights involving aircraft changes typically change to planes at adjoining or nearby gates.[3]