1.10Scale degree is more useful than absolute note

Suppose you play:

C – E – G

You can describe it as:

до – ми – соль

or:

C – E – G

But in C major it is also:

1 – 3 – 5

Now transpose that exact relationship to D major.

Start with D as 1.

The D major scale is:

D E F♯ G A B C♯

So:

1 = D

3 = F♯

5 = A

Therefore:

1-3-5 = D-F♯-A

That's D major.

And in F:

F major scale:

F G A B♭ C D E

1-3-5:

F A C

So you have discovered the general rule:

Major chord = 1-3-5 relative to its own major scale.