Equal temperament
The tuning that splits the octave into twelve exactly equal semitones — and that MIDI, nearly every synth and every DAW take for granted.
Each semitone is a frequency ratio of the twelfth root of two, roughly 1.0595. No pure fifth or pure third fits in the system: the fifth comes out at 700 cents instead of 702, and the major third at 400 instead of 386, nearly a sixth of a semitone sharp. In exchange every key sounds the same and you can modulate anywhere. Zhu Zaiyu in China computed the values with remarkable precision in 1584, and Simon Stevin reached the same result in Europe shortly after.
In software the whole thing collapses to a formula: frequency is 440 · 2(n − 69)/12 Hz, where n is the MIDI note number and 69 is the A above middle C. Everything else — just intonation, gamelan, Baroque temperaments — needs extras such as MTS or per-note pitch bend.