Joe Kagera ask for my suggestions when singing in the key of D for a baritone (male) voice.

Here I (try to) explain that all key chords (e.g. - D in the key of D) only need three notes played together to be a real chord.
You can have just two, but that is neither a major nor minor - power chords (the head bangers call 'em!).
So to make a major chord we need the 1st, 3rd and the 5th notes of the key chord.

So in D that would be D (1st, F#(3rd) and A (5th) .
Doesn't matter if you repeat them in a higher or lower - we got six strings and three notes - so we tend to.

You vocal can be the low D or the higher D.
Or the lower or higher F# ,
Or the lower or higher A.

And so that might help Joe, and of course this is the basis of basic harmony anyway, and I do offer singing mentorship like my online one-to-one guitar teaching via zoom.

Hope this helps, and apologies for not sending this out last Friday!
As always - any ads added to this video do not have my permission, nor do I support the products!