A concert pitch is the standard pitch that is defined for a certain pitch. Until well into the 20th century, very different standard pitches could be found. The instrument makers made different designs and specifications, the quality of the tuning forks for the a1 was still inconsistent up to differences of over 10 Hertz and regional musical tastes, especially in orchestras, were different.
As early as 1834, the a1 with 440 vibrations per second was standardised at the Assembly of German Naturalists and Physicians. In 1858, the Paris Academy set the diapason normal - the so-called concert pitch a1 - at 435 Hz. The pitch (440 Hz = concert pitch a1) was not defined until 1939 at an international tuning conference.
The so-called Parisian tuning of 409 Hz was used as a reference around 1900 and can be found on the soundpost of early Blüthner instruments, for example. Pianos and grand pianos from around 1910 were usually tuned to 410-435Hz - these should not be tuned higher today either, as they are sometimes not designed for 440Hz or are no longer stable with the signs of ageing. In many cases, these instruments sound best at the originally intended tuning pitch.
Nowadays, new pianos tend to be tuned higher, at 442-444Hz, especially if the instrument is used for vocal and choral accompaniment. (Source: Wikipedia)
