This equation considers two inertial frames. The primed frame is moving relative to the unprimed frame at velocity v. Fields defined in the primed frame are indicated by primes, and fields defined in the unprimed frame lack primes. The field components parallel to the velocity v are denoted by E ∥ ∥ and B ∥ ∥ while the field components perpendicular to v are denoted as E ⊥ ⊥ and B ⊥ ⊥ . In these two frames moving at relative velocity v, the E -fields and B -fields are related by: