High Caffeine Content — MPW, Carbon and building Classic Mac OS apps in OS...:

What interested me the most is how so much of the API remained identical - I was still using only functions that existed on System 1.0 in my app, but they were working just the same as ever in a Carbonized version. The single built binary ran on OS 8.1 all the way to 10.6 (care of Rosetta).

My mind wandered to Carbon as it exists in 10.10. While Apple decided not to port it to 64-bit (for all the right reasons), the 32-bit version of Carbon is still here in the latest release of OS X - I wondered how much of it was intact.

Turns out the answer is: all of it.