Due to bilateral agreements between the UK and the US, only two U.S. airlines were allowed to serve Heathrow. United and American gained the rights in 1991 from Pan Am and TWA respectively. With the arrival of Open Skies in 2008, other U.S. airline such as Delta, Northwest, and Continental inaugurated service here. American, along with BA, tried to solidify its grip on LHR by forming a compehensive alliance which was denied on both sides of the Atlantic. Even though they are One World members, the two aren't allowed to code-share or participate in each others mileage rewards programs due to monopoly concerns.