Differential signaling
The voltage and current in even and odd mode:
The characteristic impedances in even and odd mode:
The common and differential impedances:

In general, a system of N longitudinal conductors hove N1 TEM modes. So, two lines over a common ground plane have two TEM modes. The typical modes are even and odd mode.
Even mode  driven in phase with equal amplitudes
Odd mode  driven π out of phase with equal amplitudes
Note that the sign of voltage is relative to the static voltage, so that negative voltage indicates a transient from HIGH to LOW.
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If we define the voltage and current in the even and odd mode as
,then any modes can be expressed in terms of the even and odd mode:
The even and odd mode impedances are naturally defined as:
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In differential signaling, the differential voltage and current are
, then the differential impedance is double of odd mode imedance:
Similarly, the common voltage and current are
, then the common impedance is half of even mode impedance:
The impedances are depicted as follows: