To each of these quantities corresponds a definite direction, viz. that in which we have to proceed in order to make the considered quantity change in positive sense while the other three remain constant. If we denote these directions by and in the same way the directions of the coordinates by 1, 2, 3, 4, it is evident that is conjugate with 2, 3 and 4, with 3, 1 and 4, and so on; inversely 1 with ; 2 with , and so on. From what has been said above about the algebraic signs of it follows further that, if directions opposite to 1, etc. are denoted by — 1, etc., the directions — 1 and will point to the same side of an extension . The same may be said of the directions —2 and or —3 and with respect to extensions , or , while with respect to an extension , the directions 4 and point to the same side.
Finally, we shall fix (§11) as far as is necessary, which direction corresponds to three others. For that purpose we shall imagine the directions of coordinates to pass into mutually conjugate directions, which will also be called </math>, by gradual changes, in such a way that never three of them come to lie in one plane. We shall agree that after this change —4 corresponds to 1, 2, 3.
Let be the numbers 1, 2, 3, 4 in an order obtained from the natural one by an even number of permutations. Then the rule of § 11 teaches us that the direction corresponds to . It is clear that this would be the ease with , if were obtained from 1, 2, 3, 4 by an odd number of permutations. If further it is kept in mind that, always in the new case, the directions coincide with —1, —2, —3, 4, we come to the conclusion that the directions 1, 2, 3 and 4 correspond to the sets and respectively. The rule of gradual change (§11) involves that this holds also for the original case, in which 1, 2, 3, 4 were not yet mutually conjugate.
This is all that has to be said about the relations between the different directions. It must only be kept in mind, that whenever two of the first three directions are interchanged, the fourth must be reversed.
§ 23. In the neighbourhood of a point of the field-figure we may introduce as coordinates instead of the quantities defined by (19). Line-elements or finite vectors can be resolved in the directions of these coordinates, i.e. in the directions