Reciprocal space (also called -space) provides a way to visualise the results of the Fourier transform of a spatial function. It is similar in role to the frequency domain arising from the Fourier transform of a time dependent function. The domain of the spatial function itself is often referred to as real space. In physical applications, such as crystallography, both real and reciprocal space will often each be two or three dimensional. Whereas these spatial dimensions will be the same, the spaces will differ in their units, so that when the real space has units of length L, its reciprocal space will have corresponding units of one divided by length L−1 (the reciprocal of length).