The Treaty of Versailles contained an odd provision: It established a standard pitch to which orchestras could tune. For hundreds of years the agreed pitch might vary widely from one region to the next. When it became clear that the average pitch was rising over time, a French commission settled that the standard pitch for the A above middle C would be 435 hertz, and an 1885 convention of European nations adopted that as an international standard. The Treaty of Versailles in 1919 ratified that decision.
It would change again with time and technology. In the 20th century American musicians came to prefer 440 hertz, and that came to be adopted as the new standard. Today the standard tuning frequency is set by the International Organization for Standardization: ISO 16 “specifies the frequency for the note A in the treble stave and shall be 440 hertz.”