Ever since they were published in 1879, The Hanon exercises have been used by countless pianists to develop their technique, particularly in regard to the 4th and 5th fingers of each hand. There is a problem, however; all of the exercises are in the key of C (white notes only.) You can practice them for a full year and never play a black note or become familiar with any other keys. This is not good!
So, the questions arises... should you practice the Hanon exercises in all 12 keys?
As with anything else, you'll find varying points of view on this. Some teachers are all for it, while others consider it a complete waste of time.
My attitude is: If you enjoy doing this and it motivates you, why not? Just make sure you aren't just doing it to avoid practicing something that you find more challenging, like playing chord voicings in all keys or memorizing jazz tunes.
I spent a few years practicing Hanon in all keys, and loved every minute! (In fact, it was my jazz piano teacher at the time, Harold Danko, who recommended that I do this.) After you get really familiar with each key, try improvising simple melodies in some of the less common keys. You'll be glad you did this every time you come to the end of the bridge to "All The Things You Are," when it goes to E major. And, if you play with rock guitarists, you'll become very comfortable playing in all the "guitar" keys such as D, A, and E!
The whole musical journey is about having fun, practicing what you need to practice, playing music with others, and growing as a pianist. If playing the Hanon exercises fulfills any of these functions, then I say "go for it!"
Good luck 🙂
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