The most common form of the transformation, parametrized by the real constant v , representing a velocity confined to the x -direction, is expressed as t ′ = γ γ ( t − − v x c 2 ) x ′ = γ γ ( x − − v t ) y ′ = y z ′ = z where (t, x, y, z) and (t ′, x ′, y ′, z ′) are the coordinates of an event in two frames with the origins coinciding at t = t ′ =0, where the primed frame is seen from the unprimed frame as moving with speed v along the x -axis, where c is the speed of light, and γ γ = ( 1 − − v 2 c 2 ) − − 1 is the Lorentz factor. When speed v is much smaller than c, the Lorentz factor is negligibly different from 1, but as v approaches c, γ γ grows without bound. The value of v must be smaller than c for the transformation to make sense.