Why do keyboard instruments belong to several different sound-production categories?

Keyboard instruments are grouped together by musicians based on their shared interface—a series of keys, buttons, or levers used to trigger sound—rather than the specific physical mechanism that produces the vibration.12

In contrast, the Sachs-Hornbostel system, which is the standard scientific method for classifying instruments, categorizes them by their primary vibrating medium. Because different keyboard instruments utilize entirely different methods to create sound, they fall into several distinct categories within this system:1

• Chordophones: Instruments where sound is produced by vibrating strings, such as the piano (struck strings) and the harpsichord (plucked strings).134
• Aerophones: Instruments where sound is produced by a vibrating column of air, such as the pipe organ, which uses flue pipes (whistle flutes) and reed stops.15
• Idiophones: Instruments made of sonorous material that vibrates to produce sound, such as the celesta, which uses hammers to strike metal bars.123
• Electrophones: Instruments where vibration is produced by oscillating electric circuits, such as synthesizers and electronic organs.123

Ultimately, while the keyboard provides a unified way to play these instruments, their underlying acoustic principles are diverse.13