Hello everybody,

Today I was showing a friend some chords I learned here on the page. While I was playing the chords, like C Major and explaining that it was a C Major, he said that the name was "Do"

When I played a D Major, he said that was a "Re"

So he knows the chords by Do, Re, Mi, but I learned them here as C major, D, major so my question is, which way is correct? and if both are, then why are we learning them like that only if both names are possible?

Thanks everybody