9 String Theory is not just a catchy name but also a straightforward calculation. It’s a musical duo featuring Angelina Galashenkova on the domra, which has three strings, and John Huston on classical guitar, which has six strings.
The domra is a Russian folk instrument, or as Galashenkova said, “I describe it as the Russian lute, Russian mandolin.”
Classical guitar and domra might be a strange pairing in the United States, but in Russia, it is more common. Usually, the domra takes the melody, while the guitar is on the harmony part.