Why Is the Thumb Not Absolutely Forbidden on Black Keys?

Adult learners are often told, “Don't put your thumb on a black key.” It is a useful starting principle, but not a bodily prohibition. Black keys are higher, shorter and narrower than white keys. In a continuous scale, sending the thumb onto a black key can draw the hand deep into the keyboard and enlarge the sideways movement needed to return to the whites. That is why the B-flat major scale normally places the thumb on white keys and organises crossings more smoothly.

Music, however, is not an endless one-way scale. A right-hand chord with F-sharp at the bottom, a melody lying mainly on black keys, or a contrapuntal passage that must preserve one voice may make the thumb on a black key the more natural choice. Professional teaching sources present avoidance as a general rule while explicitly allowing exceptions. The useful question is not whether a rule has been broken, but whether the fingering twists the wrist, interrupts the phrase, obstructs the next position or becomes unreliable near performance tempo.

Do not test only the isolated note. Join one or two beats on either side and compare two fingerings both slowly and near the intended speed. Listen for an accidental accent and notice whether the hand can continue naturally into the next group. Once a more coherent solution has been chosen, keep it stable; frequent changes force the movement sequence to be decided afresh each time. A principle narrows the options, but the relationship between music and the body determines the answer.

When practising such a passage in <a href="https://apps.apple.com/us/app/scoreflow/id6796205656">ScoreFlow</a&gt;, test the thumb on the black key together with the surrounding bars. If sound, pulse and repositioning become more stable, there is no need to create a detour merely to obey a slogan.

https://hub.yamaha.com/keyboards/k-how-to/the-basics-of-piano-keyboard-fingering/
https://www.pianistmagazine.com/finding-the-appropriate-fingering/