What modulation method is most closely related to frequency modulation?

Phase Modulation (PM) and Frequency Modulation (FM) are mathematically and physically intertwined, both falling under the broader category of 'angle modulation'. Modulating the phase of a carrier wave inherently shifts its frequency, and modulating its frequency inherently shifts its phase. Because of this direct relationship, a standard FM receiver can easily demodulate a PM signal. In fact, many modern amateur radio transceivers do not use a direct frequency oscillator to generate an FM signal. Instead, they pass the audio through a phase modulator to indirectly create the frequency modulated signal we transmit.