Here, v is the parameter of the transformation, for a given boost it is a constant number, but can take a continuous range of values. In the setup used here, positive relative velocity v > 0 is motion along the positive directions of the xx ′ axes, zero relative velocity v = 0 is no relative motion, while negative relative velocity v < 0 is relative motion along the negative directions of the xx ′ axes. The magnitude of relative velocity v cannot equal or exceed c, so only subluminal speeds − c < v < c are allowed. The corresponding range of γ is 1 ≤ γ < ∞.