system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 in music why does the DO note correspond to C and not to the first letter of the alphabet?
gpt
The A-B-C-D-E-F-G notation system dates to the 6th century AD, at which point it the Aeolian mode (analogous to the modern-day natural minor scale) was one of the most common in Western music - and certainly more common than the Ionian (or major) mode.

The solfege/Do-Re-Mi system dates from the 11th century, at which point the Ionian mode was gaining in popularity. The names derive from the syllables on each note in a popular hymn at the time, *Ut queant laxis*.

All the other answers so far are referring to the moveable-Do system, but they're useless for your question, which is referring to the fixed-Do system, as used in the Romance and Slavic languages.

**ELI5 version**: The "A-B-C" system is older than the "Do-Re-Mi" system. The "A-B-C" system was designed around the minor scale, and the "Do-Re-Mi" system was designed around the major scale.