Lorentz transformations can be parametrized by rapidity φ for a boost in the direction of a three-dimensional unit vector n ^ ^ = ( n 1 , n 2 , n 3 ) , and a rotation angle θ about a three-dimensional unit vector a ^ ^ = ( a 1 , a 2 , a 3 ) defining an axis, so φ φ n ^ ^ = φ φ ( n 1 , n 2 , n 3 ) and θ θ a ^ ^ = θ θ ( a 1 , a 2 , a 3 ) are together six parameters of the Lorentz group (three for rotations and three for boosts). The Lorentz group is 6-dimensional.