The prerequisite for the necessity of operating with two planes must be that the two planes, when they are tentatively set up, cannot be shown to have the same structure throughout, with a one-a-one relation between the functives of the one plane and the functives of the other. We shall express this by saying that the two planes must not be ‘conformal’. Two functives are said to be conformal if any particular derivate of the one functive without exception enters the same functions as a particular derivate of the other functive, and vice versa.
- Hjelmslev (1961), a pag.112