17 steel keys and two vibrato holes on the back.
The thumb piano originates in Africa. It is also known as the mbira, kalimba or likembe. The metal keys are mounted on a wooden box that acts as the resonating chamber.
To play, rest the thumb piano on the fingers of both hands and hold it between your palms. Cover the vibrato holes with your fingers. Then pluck the keys with your thumbs. As you play you can un-cover and cover the vibrato holes as you like. Many different tunings are acceptable, according to personal preference. Specific tuning of a thumb piano is only an issue when two instruments are played together; then they should generally agree in tuning. Otherwise, the tuning is subjective and should only be pleasing to the player.
The longer keys have lower pitches and shorter keys have higher pitches. The keys can be adjusted by tapping them in or out to adjust their length, and therefore their pitch.