The lecture presents a general framework to interpret the concept of Instantaneous Frequency (IF) in three-phase systems. In the first part, the conventional frequency-domain analysis based on the Fourier transform as well as the definition of IF, which is based on the concept of analytic signals and Hilbert Transforms. The link between analytic signals and Clarke transform of three-phase voltages of an ac power system is also shown. Then the well-known five paradoxes of the IF are stated. In the second part of the lecture, an approach based on a geometric interpretation of the frequency is proposed. This approach serves to revisit the five IF paradoxes and explain them through a common framework. Finally, it is shown how the geometric approach and the properties of geometric invariants allow generalizing the Park transform. A variety of illustrative examples show the features of the geometric approach, and its practical applications are discussed based on IEEE benchmark systems.