For some of the cases we looked at today, one of the op-amp inputs was tied to ground, which made the other a ``virtual ground''. But Rule #2 still works even if one of the inputs isn't ground, and the rule can help analyze the circuit behavior. An example is the non-inverting amplifier, for which the op-amp's inverting input voltage is given by the feedback voltage divider. If we use Rule #2, we can calculate the gain.