In the second form given above, the phase is often generalized to , by replacing the wavenumber "k" with a wave vector that specifies the direction and wavenumber of a plane wave in 3-space, parameterized by position vector r. In that case, the wavenumber "k", the magnitude of k, is still in the same relationship with wavelength as shown above, with "v" being interpreted as scalar speed in the direction of the wave vector. The first form, using reciprocal wavelength in the phase, does not generalize as easily to a wave in an arbitrary direction.