The Hammond organ was a masterpiece of electro-mechanical engineering. Instead of pipes, it used 'tonewheels'—rotating metal discs with notched edges. As these discs spun near an electromagnetic pickup, they created an alternating current that was converted into sound. A standard Hammond B3 has 91 tonewheels spinning at once. The player uses 'drawbars' to control the volume of different harmonics, allowing for millions of possible tonal combinations. This 'additive synthesis' paved the way for modern electronic music.