The reason why instrumental music is calibrated with one tone is because the height of one tone in music has a certain standard. The international standard height is the A sound that vibrates 440 times per second, that is, the A sound in a small group is the "standard sound". In a music system, a tone is at the top of the basic level, so correcting a tone helps to determine other pitches in the music system.

In addition, with the passage of time, the orchestra continued to expand, and other kinds of musical instruments (such as woodwinds and brass pipes) also joined the orchestra, and followed the practice of string bands, taking one voice as the standard for calibration. Even if the orchestra uses the "A" of the keyboard instrument to tune, the oboe (or the first violin) will still pick up the instrument for other members of the orchestra and play the A note to make sure that everyone can hear it.

Therefore, the sound calibrator usually takes the A sound as the reference sound.