Microtonal music or microtonality is the use in music of micro | Classical Guitar World

Microtonal music or microtonality is the use in music of microtones—intervals smaller than a semitone, also called "microintervals". It may also be extended to include any music using intervals not found in the customary Western tuning of twelve equal intervals per octave.
Here is the example of microtonal guitar. Tolgahan Çoğulu designed his "Adjustable Microtonal Guitar" in 2008. In designing Adjustable Microtonal Guitar, Tolgahan Çoğulu was inspired by René Lacote's and Walter Vogt's guitars. On the Adjustable Microtonal Guitar's fretboard, all the frets are movable in the channels under each string. Besides, any number of frets can be inserted into or removed from the fretboard.
Enjoy!