This question seems simple, but answering it is surprisingly ambiguous. This is because two different sets of criteria—the method of performance and the principle of sound production—apply simultaneously. Keyboard instruments are categorized by their method of performance, meaning instruments operated through a keyboard. Because of this characteristic, the piano is recognized as the most representative keyboard instrument. On the other hand, string and percussion instruments are categorized by how the sound is produced. That is, if sound is made by the vibration of strings, it is a string instrument; if sound is generated by striking an object to create vibration, it is a percussion instrument.
 

#. Sound Unexplained by Classification

According to these criteria, the answer is relatively clear. The piano is a keyboard instrument played via keys, and at the same time, it belongs to the string family because it produces sound through vibrating strings. More accurately, it is classified as a "struck string instrument" (chordophone) because hammers strike the strings to produce notes. The issue of classification ends here. However, simply having a classification doesn't fully explain the piano we actually hear. There is a gap between the mechanical structure of the instrument and our auditory experience.
 

#. Fading Sound

The moment a piano key is pressed, a hammer strikes the string, and the sound starts strongly before gradually fading away. This mode of sound production—initiated by an impact and then decaying—is closer to the mechanics of a percussion instrument. Consequently, each note exists independently, and its duration is limited. This characteristic was the same for early keyboard instruments that predated the piano. While the clavichord and harpsichord manipulated strings differently, they shared the commonality with the piano that once a sound began, it weakened over time.
 

#. The Device That Makes the Difference—The Pedal

Despite these physical characteristics, the performances by pianists we actually hear feel different. Melodies connect as if drawing a line, and harmonies form a rich acoustic structure through the resonance of overlapping notes. This cannot be explained by percussion characteristics alone. Why does this difference occur? The element that creates that difference is the piano "pedal." When you press the right pedal (damper pedal), the sound does not disappear even after you release the key; it sustains. The sounding note and the strings with frequencies close to its overtones vibrate together, causing the sound to expand and become richer. Additionally, as the previously played sound and the newly pressed sound naturally overlap in time, we hear a continuous flow rather than isolated notes. Most people think of pedaling as a binary choice of "pressing" or "releasing." However, in actual professional performance, the depth and timing of the pedal are adjusted continuously. At some moments, the pedal is pressed deeply to broaden the resonance; at others, it is pressed very shallowly to let the sound connect just slightly. Furthermore, whether the pedal is pressed before hitting a note or changed after the sound is produced alters how the resonance connects, significantly impacting the music's flow and transparency. For this reason, pedaling cannot be described as a simple foot movement. What is more important for the performer is not the movement of the foot itself, but how they listen to and judge the sound.
 

#. The Ear That Decides the Pedal

There is something my professor always emphasized during piano lessons: "The decision to pedal is not made by the foot, but by the ear." Even for the same piece, the resonance changes every time depending on the performance space, the condition of the piano, and the performer's touch. Therefore, the pedal cannot be used in a fixed way. The performer constantly judges with their ears how long the current sound should be maintained and at what moment the previous resonance should be cleared. This is where the difference in performance is revealed. Indeed, I have often experienced this difference while teaching students. Students who play without listening view the pedal only as a means for connection and richness, holding it down indiscriminately for too long. As a result, clashing harmonies mix together, resulting in the mistake of creating a muddy sound. On the other hand, some students use the pedal to control the amount and duration of resonance, selectively leaving only the necessary sounds to organize the acoustics. What is interesting is that while we easily marvel at a performer's flashy and fast hand movements, we rarely pay attention to the subtle movements of the feet that significantly affect the sound. Perhaps that is the scene that best captures the essence of the piano as an instrument.
 

#. The Unseen Completes the Sound

The piano is an instrument that is difficult to define simply by a single standard. It is a keyboard instrument, a string instrument, and at the same time, it possesses the character of a percussion instrument. Within these complex traits, the sound begins at the fingertips, but its completion is achieved at the toes. And what moves those feet is ultimately the ear. I encourage you to feel the subtle movements of the feet that we often fail to recognize behind the flashy hand movements the next time you have the opportunity to listen to a piano performance.