When did 440 Hz become standard?

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Tags

1. #society
2. #reached
3. #after
4. #johann
5. #industry
6. #using
7. #instrument
8. #scheibler
9. #began
10. #informal
11. #measure
12. #history
13. #natural
14. #pitch
15. #heinrich
16. #manufacturing
17. #tonometer
18. #music
19. #american
20. #recommended
21. #german
22. #approved
23. #standard
24. #inventing