The Kaeoubo (kA-ooh-bO) is an Avotoc musical instrument that sonically resembles an acoustic guitar. Unlike the acoustic guitar of real-world, the Kaeoubo only has four strings and uses the concept of the harmonic series and its partials as nodes rather than frets. The four strings are secured inside and outside the instrument, which has a gourd like shape. On the outside of the instrument the strings are plucked and held in determinable positions to change the way the string vibrates inside the gourd to produce sound. The saddle on the top of the instrument is rounded to maximize the amount of resonance that moves through the string from the outside of the instrument inward. Inside the instrument, where the strings are attached, are tuning pegs so that the notes that each string produces can be changed.
The Kaeoubo sprang from playing a guitar in standard tuning with none of the strings depressed on a fret. All Avotoc music is produced in this manner, with key changes being made possible by the use of a capo. The four strings came as a result of the pitches present on standard tuning (EADGBE) and the instrument’s “use” of the harmonic series. I have no idea how it would work physically, but violinists can use harmonics so it’s not impossible per se.