The inverse relations (t, x, y, z in terms of t ′, x ′, y ′, z ′) can be found by algebraically solving the original set of equations. A more efficient way is to use physical principles. Here F ′ is the "stationary" frame while F is the "moving" frame. According to the principle of relativity, there is no privileged frame of reference, so the transformations from F ′ to F must take exactly the same form as the transformations from F to F ′. The only difference is F moves with velocity − v relative to F ′ (i.e., the relative velocity has the same magnitude but is oppositely directed). Thus if an observer in F ′ notes an event t ′, x ′, y ′, z ′, then an observer in F notes the same event with coordinates