Phase

Phase is each partial's offset within the waveform's cycle — where in the loop every sine component starts — measured as an angle rather than a time.

Two frames can share identical partial amplitudes yet look completely different because their partials are phased differently; the ear keys mostly on the spectrum, but the shape differences matter the moment frames interact. Phase agreement is what lets a morph stay loud and coherent, and out-of-step phases are the usual reason a plain crossfade dips or turns hollow halfway through.