This guitar-like instrument uses stretchy strings. Among the available options for elastic string materials, none can take high tension remotely as well as the usual steel or nylon, so this necessarily had to be a low-tension instrument. The string length is only 3/4 that of a typical guitar, and the strings are tuned an octave lower than standard guitar tuning. Even with that it was difficult to find elastic string materials able to take the tension of the highest string. I ended up using several different materials across the six strings to get the needed combination of good tone and sufficient strength. The pickup is a special configuration made for elastic strings, as described here. The elastic string sound is soft and warm, with lots of fundamental and virtually no overtones. This instrument can be played with finger-picking styles similar to what you’d do on a standard guitar (with, admittedly, a tendency to sound muddy if you get into much complexity in the low end), or you can just treat it as a six-string bass. For more on elastic strings, see here.
In the video linked below, Elastic Guitar is one of several guitar-like instruments discussed.