system
Context: A440 or A4 (also known as the Stuttgart pitch), which has a frequency of 440 Hz, is the musical note of A above middle C and serves as a general tuning standard for musical pitch. 

The International Organization for Standardization classify it as ISO 16. Prior to the standardization on 440 Hz, other frequencies were standardised upon. Although not universally accepted, it serves as the audio frequency reference for the calibration of acoustic equipment and the tuning of pianos, violins, and other musical instruments. 

Prior to the standardization on 440 Hz, many countries and organizations followed the French standard since the 1860s of 435 Hz, which had also been the Austrian government's 1885 recommendation. Johann Heinrich Scheibler recommended A440 as a standard in 1834 after inventing the "tonometer" to measure pitch, and it was approved by the the same year. 

The American music industry reached an informal standard of 440 Hz in 1926, and some began using it in instrument manufacturing. In 1936 the American Standards Association recommended that the A above middle C be tuned to 440 Hz. This standard was taken up by the International Organization for Standardization in 1955 (reaffirmed by them in 1975) as ISO 16. Although not universally accepted, since then it has served as the audio frequency reference for the calibration of acoustic equipment and the tuning of pianos, violins, and other musical instruments.
human
What entity standardized A4 on 440 Hertz?
gpt
International Organization for Standardization
human
What do they classify it as?
gpt
ISO 16
human
Was A4 always standardized at 440 Hertz?
gpt
no
human
What is one instrument A4 is used to tune?
gpt
pianos
human
And another?
gpt
violins
human
Do they mention tuning guitars?
gpt
no
human
What is A4 also called?
gpt
A440
human
Or what else?
gpt
Stuttgart pitch
human
What note is it?
gpt
A above middle C
human
In the 1920s, did the American music companies have A4 as an official standard?
gpt
no
human
Who made a recommendation in 1936?
gpt
the American Standards Association
human
What did they recommend?
gpt
that the A above middle C be tuned to 440 Hz.
human
What happened in 1955?
gpt
it was used by the International Organization for Standardization in 1955
human
When did they reiterate their support for A4?
gpt
1975
human
Does everyone agree with the standard?
gpt
. no
human
What did Scheibler invent?
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the tonometer
human
What did it do?
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measure pitch,
human
When did he create it?
gpt
1834
human
How many Hertz was the French standard?
gpt
435 Hz, w
human
Since when?
gpt
1860