Tuning pianos to A440 has become the norm in modern times. The note A above middle C is tuned to vibrate at 440 Hz (440 vibrations per second), and the piano’s other notes are tuned in relation to it. All pianos in the world can be tuned to the same pitch because A440 is a measurable standard.
It wasn’t always like this. The International Organization for Standardization (ISO) decided in the 1950s that A440 would be the standard frequency for the note A above middle C on pianos and other musical instruments. Pianos all over the world can be tuned to this internationally recognized standard pitch, ensuring that the same note played on any pair of properly tuned pianos sounds the same.
Prior to this widely accepted standard, pianos and other musical instruments could not be tuned to a single common pitch. Pitch pipes or tuning forks could often vary in pitch by a significant amount, allowing even the most inexperienced musician to distinguish between two pianos playing the same note. Instruments played in the same venue, such as the piano or organ, would all be tuned to the same pitch, but the A note on one piano might vibrate at 445 Hz, while the A note on another piano might vibrate at 425 Hz.
The fact that pianos are all tuned to the same note is a great benefit to audiophiles, especially given the quality of audio recordings. Trying to play along with a recorded performance on a piano tuned to a different pitch than the piano in your den would be excruciating. Two pianists playing a duet on different pianos can rest assured that their instruments will blend well because all of their notes will be pitched correctly.
Tuning a piano is a delicate process that necessitates the services of a professional piano tuner. With the standard A440 tuning, whether she uses an electric tuner or a simple tuning fork, all the pianos she tunes will be the same pitch. A qualified piano tuner will be able to tune the rest of the keys by ear based on that note, and all pianos will sound the same based on the standard pitch of the first string.