MIDI Tuning Standard (MTS)
A way to tell a synth exactly which frequency each MIDI note should have, rather than assuming equal temperament.
MIDI assumes twelve equal semitones per octave, which suits a piano but not Arabic maqam, gamelan, just-intoned chords or Baroque temperaments. MTS, added to the specification in 1992, lets a sender transmit a tuning table over SysEx in which each of the 128 notes gets its own frequency. Resolution is 14 bits within the semitone, roughly a sixtieth of a cent — far below anything anyone can hear.
There are bulk dumps of whole tables as well as real-time changes to single notes. Hardware support has been patchy, and many microtonal-capable synths do it through their own menus instead. In the DAW world, ODDSound's MTS-ESP has become a common way to let one central plugin retune every instrument in a project at once.