system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are different instruments tuned to different keys?

A violin is tuned to the key of C.  A french horn is tuned to the key of F (sometimes).  What does this actually mean? Does an F played by a french horn sound the same pitch as a C played on a violin? If so, how is that possible, given that they (presumably) have the same frequencies? If not, what does it actually mean, and why are the instruments like that?
gpt
This is called transposing. It is done mainly for wind and brass instruments to keep fingerings consistent.

Transposing instruments are said to be "tuned to a key". If a clarinet is tuned to A, then when the clarinet plays a written C the sound produced will be an A.