In a circuit which has a reference point (or ground point), most voltages will
be expressed with respect to the reference point. In such a case it is no
longer necessary to express a voltage using a dual subscript. Rather if we
wish to express the voltage at point a with respect to ground, we simply refer
to this as Va. Similarly, the voltage at point b would be referred to as Vb.
Therefore, any voltage which has only a single subscript is always referenced
to the ground point of the circuit.