Additive synthesis
Building a sound by summing sine waves, one per partial — Fourier run in reverse.
Any periodic tone can be described as a sum of sine waves, each with its own frequency and level. Additive synthesis takes that literally: give every partial its own oscillator and envelope, and in principle any timbre can be rebuilt. The Hammond organ's drawbars from 1935 are a mechanical version with nine fixed harmonics.
The catch is the count. A convincing piano note needs dozens of partials, each changing over time, which is a lot of knobs for a musician and a lot of arithmetic for a 1980 computer. The Kawai K5 offered 126 harmonics in 1987 and sold modestly. So subtractive and FM synthesis won, since they coax rich spectra out of few parameters. Cheap computation and FFT-based resynthesis have since given the technique a second life in software.