"Major alliances" refers to significant partnerships or Coalitions formed by countries, groups, or organizations that work together towards a common goal or for mutual benefit. Full definition
And there are also individual partner airlines outside of major alliances that you can book with your miles. (rewardexpert.com)
Airlines from all three major alliances fly to both of those cities. (rewardexpert.com)
While you still have access to all three major alliances with ThankYou, you'll have to book your tickets through the smaller foreign carriers, which could result in poorer redemption rates and / or less award availability on routes from the U.S. (rewardexpert.com)