I have a friend who lost his left hand in an accident. He is a good piano player and can play good music with his right hand. I want to build a robotic machine which will be able to play notes played by the left hand. But the problem is I am not a musician, I don't know how music is composed, and I live far from him so I am can't ask him how things work. I have a basic understanding of what notes are and how they work in a mathematical sense because of a course I took on "Signal and Systems" in engineering. So, here are my questions:

What is the use of the notes played by left hand? Is it in some way a derivative of what's being played with the right hand? Can the left hand be completely independent of what's being played with right hand?

  • 1
    A question which may be relevant - does he play only an acoustic (real, mechanical) piano or a digital one of some sort? Because if it's a digital it might be easy send note commands electronically - passing in MIDI signals or similar. (Of course if it's a pure acoustic piano this won't be possible.)
    – Andy
    Jul 28, 2016 at 14:28
  • 1
    @Andy It is possible to play a "pure acoustic piano" automatically. Mechanical player-pianos were first built more than 100 years ago. There are modern top-quality acoustic pianos which have electrical actuators that operate the keys directly, to record and reproduce a human performance - some of these instruments divide the motion of the piano key into as many as 1024 increments to record and reproduce it accurately.
    – user19146
    Jul 28, 2016 at 15:05
  • Not an answer, but a thought that another option would be to have a set of foot pedals that would trigger perhaps a sampled piano (there are MIDI mechanical pianos, if price and complexity are not concerns). Of course any time the damper pedal is used by one foot that only leaves one foot for bass notes, and doing chords with the foot is possible with some MIDI processing. I thought I'd throw this in as an off-the-shelf idea, either temporary or permanent. Jul 28, 2016 at 17:00
  • Does your friend read music? Classical or sheet music? Does he improvise? Any particular style? You might check out the accompaniment functions of electric organs built for home/hobby use, for ideas. They are mostly geared to playing pop tunes/show tunes. Some play rather elaborate patterns when supplied the root note of the chord to play. If playing accompaniments to songs or improvising, the foot could specify a root note and maybe the "left hand" could select from a limited number of patterns, or some such. But this wouldn't work for playing a Beethoven Sonata. Jul 28, 2016 at 17:46
  • you're an awesome friend @HappyFeet
    – sova
    Jul 29, 2016 at 16:58

6 Answers 6


In various settings, the left hand can do everything that the right hand can do.

When musicians first begin playing the piano, it's typically with music where the melody is in the right hand and the chords are in the left hand. As a player becomes more advanced, the melodic line will start to switch between hands. When the player becomes really advanced, the melodic line will even switch between different voices within a single hand!

So, the use of the notes played by the left hand? Anything and everything!

Is it in some way a derivative of whats being played with the right hand? Typically, but it's more of a symbiotic relationship. The hands are related to each other, and one can't always say which is a derivative of which, because it's ultimately a chicken and egg situation: which came first, the harmony or the melody? (Composers and theorists throughout history have had different answers to that question.)

Can the left hand be completely independent of what's being played with the right hand? Absolutely!

You might also consider looking at one-handed piano repertoire as a gift. (Although this repertoire is admittedly usually written for the left hand alone, alas.)


You don't need to reinvent the wheel here - you can already buy solutions. The cheapest option (a few hundred dollars) would be a good quality piano-action MIDI keyboard with the sound generator that is "played" partly by your friend and partly by a computer. At the top end of the price range (up to a few hundred thousand dollars) instruments like the Yamaha Disklavier or the Steinway Spirio are top quality acoustic pianos with built-in electrical actuators that work by moving the keys.

Designing and building "piano playing robot" could be an interesting and challenging project, but you are unlikely to be able to do it better or cheaper than the commercial products already out there.

If you want to try the project anyway, I would recommend starting by learning to play a keyboard yourself before you start trying to specify what the device should do.


Something that requires a bit of tinkering on the software side, would be a Myo Gesture. This can pick up muscle signals, and you can train it to perform certain things. In this particular case you could train it to send midi notes (https://en.wikipedia.org/wiki/MIDI), which is a certain format for signals that virutally all keyboards and music-software understands. So he could play a range (say 5-10) notes with different muscle movements. This device is in general powerful, I have seen people that lost a limb controling mechanical hands with it. In your very specific application you could just skip the mechanical device inbetween and just control the music with the arm.

Requirement : Investment in the device, Programming Experience, a bit of time to learn and adapt the gestures with the user.

Hope this can help a bit.

  • Expanding a bit: anyone can play a duet with a recording of the left hand part, and that includes playing along with MIDI recordings of piano music. (There are Yamaha digital pianos around $500-600 with weighted keys that sound great and will play MIDI.) However, the recorded part is inflexible. The device which reads impulses from muscles could serve the role of influencing the tempo and volume of the playback of the pre-recorded part via timing and strength of a tapping of some sort, allowing the performer to have more artistic control and ownership of the result. Jul 28, 2016 at 17:37

Many keyboards come with an auto-harmonization feature that will play chords or arpeggios when a key is pressed on the left side of the keyboard. These usually only work if the entire piano piece is in one scale as the scale needs to be set beforehand. Assuming your friend could play the equivalent of one-finger piano in the left hand, this might be the easiest solution.


I know your question, design some software, or some method to replace the left hand. But, your friend's music can be natural and not sound mechanical, or contrived. Another thought for you. Guitarists use the left hand for chords and the right hand for melody to produce beautiful music. I think this can be done on the piano using only the right hand to do both chords and melody.

So, as an alternative, studying the methods of guitarists and applying it to piano may work for your friend, not such a bad idea and I like that it makes his music his signature. One, it will give your friend a unique style. Two, it will work musically. It sounds like your friend may already at doing this? Let the natural develop, much better. I think a unique style is always welcomed in music. Think like us guitarists.


There are established methods of mechanising or pre-recording the notes the LH would have played. But I imagine you're looking for a way your friend can actually PLAY the music.

The LH notes are not derived from the RH ones. The two hands are completely independent. Which, I think, counts out some of the approaches that were in your mind.

"I live far from him so I am can't ask him how things work." So call him, email him... You're thinking of starting an engineering project without experience of the application and without consulting the end user. Are you mad? :-)

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