Rx Half-Plane:
Remember the quarter-planes for resistance R and inductive reactance XL from the previous parts. This is similar as the upper-right quadrant of the complex-number plane. Likewise, the quarter-plane for resistance R and capacitive reactance XC is similar as the lower-right quadrant of the complex-number plane. Resistances are symbolized by non-negative real numbers. Reactances, whether they are inductive (i.e., positive) or capacitive (i.e., negative), correspond to imaginary numbers.
There is no such thing, harshly speaking, as negative resistance. That is to say, one cannot have anything superior than a perfect conductor. In some situations, a source of dc, like a battery, can be treated as a negative resistance; in other situations, a device can behave as when its resistance was negative under certain changing conditions. Usually, though, in the RX (resistance-reactance) half-plane, the resistance value is non-negative (the figure is as shown below).
Figure: The RX impedance half-plane showing few points for particular complex-number impedances.