title_body: Why do a quadcopter's opposite rotors spin in the same direction?
upvoted_answer: This design is to prevent unwanted yaw when moving, or vice-versa.
  
  Consider the following two actions:
  
  
  When a quadcopter yaws, it does so by creating a speed difference between the opposite-rotating rotors; creates a torque that rotates the quad.  
  When a quadcopter moves left/right, it does so by slowing all the motors on one side of the frame and/or speeding those on the other, imparting a slight roll - the thrust is now slightly sideways, so it moves. (This is the same for front/back.)
  
  
  From this, you can infer that if the same-direction rotors were on the same side as each other, when creating a speed difference to yaw the aircraft you would also impart a roll. Similarly, when trying to roll the aircraft would yaw.
  
  The image below shows visually how an opposite-corner rotation quadcopter moves: