Sometimes it is more convenient to use β = v / c (lowercase beta) instead of v, so that c t ′ = γ γ ( c t − − β β x ) , x ′ = γ γ ( x − − β β c t ) , which shows much more clearly the symmetry in the transformation. From the allowed ranges of v and the definition of β, it follows −1 < β < 1. The use of β and γ is standard throughout the literature.