Differential pairs of line have two points to note,
One is the length of the two lines to the same length,
The other is the distance between the two lines (this distance determined by the differential impedance) to remain the same,
That is, to keep parallel.There are two parallel ways, one for the two lines walking in the same line layer (side-by-side),
one for the two lines in the upper and lower adjacent two layers (over-under).
Generally in front of the side-by-side to achieve more ways.