Additive synthesis
Build any sound by stacking sine waves — the approach the very first synth used in 1897.
Additive synthesis is the mirror image of subtractive: instead of carving harmonics away from a rich wave, you build a sound up from nothing by stacking pure sine waves, one harmonic at a time. It is the oldest idea in the book — the 1897 Telharmonium worked exactly this way.
Each slider above is one harmonic — a sine wave at a whole-number multiple of the base pitch (2×, 3×, 4×…). Bring up just the first and you have a pure sine. Add the odd harmonics and it turns hollow and reedy; add them all and it brightens. Watch the wave in the display change shape as you rebuild it from sines.
This is exactly how a drawbar organ works: nine sliders, each a harmonic, and the mix you pull is the sound. There is no filter and no envelope shaping the tone — the recipe of sines is the whole instrument.