Around 1700, an Italian instrument builder by the name of Bartolomeo Cristofori (1655-1731) invented an instrument and called it Arpicembalo. It would become arguably the most important instrument of modern times.
Somehow the name wasn’t catchy enough and Cristoforis description of the instrument, which included the word piano (Italian for soft), slowly became the actual name. There are only 3 Cristofori pianos known to still exist and only one is playable.
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Information sources:
https://en.wikipedia.org/wiki/Bartolomeo_Cristofori
http://www.britannica.com/biography/Bartolomeo-Cristofori
Photo Credits / Sources:
By Shriram Rajagopalan (Flickr: Met-32) [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], undefined