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How to ? E major chord in C major

Let’s say I am in C major and in my progression I want a C major chord ( I ) followed by a E major chord. How do yo do that ?

Input V/vi (click on the ‘V/’ button in the chord properties panel then click on ‘V/vi’):

OK, thanks, it works. I am lost theoretically speaking.
What does “secondary” mean in that case, and what does V/vi mean ?
I know about the seven degrees, I know my scales, I know the basic alternance of major / minor chords in a scale. But this is unknown to me.

V/vi is read as ‘V of vi’ because it literally is the V chord of the vi chord

To get V/vi in C major, you take the vi chord, Am, then take the V chord in the major scale of that chord. So that means you take the V chord in A major which is E. So in C major the V/vi chord is E

This works for the other diatonic chords as well, e.g. V/V in C major is D:

the V chord in C major is G
the V chord in G major is D

It also works for non-diatonic chords, e.g. V/bVI in C major is Eb:

the bVI chord in C major is Ab
the V chord in Ab major is Eb

That helps a lot. Quite tricky and mind boggling but I do get the principle.
Thanks a lot.