Concert pitch is the standard tuning pitch defined for a particular note. Well into the 20th century, very different standard pitches were found. Instrument makers used different designs and specifications, the quality of the tuning forks for A4 was still inconsistent, with differences of more than 10 hertz, and regional musical tastes, especially among orchestras, differed.
As early as 1834, the assembly of German natural scientists and physicians proposed A4 at 440 vibrations per second as a uniform standard. In 1858, the Paris Academy set the diapason normal – the so-called concert pitch A4 – at 435 Hz. The pitch (440 Hz = concert pitch A4) was not fixed until an international pitch conference in 1939.
The so-called Paris pitch of 409 Hz was used as a reference around 1900, as can be found, for example, on the pinblock of early Blüthner instruments. Upright and grand pianos from around 1910 were usually tuned at 410–435 Hz – they should not be tuned higher today either, as they are sometimes not designed for 440 Hz or no longer hold stably due to signs of aging. In many cases, these instruments also sound most beautiful at the originally intended pitch.
New pianos today tend to be tuned higher, at 442–444 Hz, especially if the instrument is used to accompany singers and choirs. (Source: Wikipedia)
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