DADGAD & Irish music
Before starting to play Irish music, it's important to make a point about how to build a chord.
A chord is made up of three notes: first/root (1) + third major or minor (3) + fifth (5).
An example for the scale of D major (ionian):
​
​
1 - First = Ré
2 - Second = Mi
3 - Third = Fa#
4 - Fourth = Sol
5 - Fifth = La
6 - Sixt = Si
7 - Seventh = C#
So for a D major = 1 + 3 + 5 = D + F# + A or D minor = D + F + A
Then we can add some notes:
For a D major 7th = 1 + 3 + 5 + 7 = D + F# + A + C# etc...
​
We can also remove the third to get a chord of two notes, we call them "poor" or "without third", or "modal" (written "5" in the PDF chords of the menu DADGAD).
So for a D modal (D5) = 1 + 5 = D + A
These chords are very important because they are very useful in traditional music. They are neutral, because there's no third, so we can play them in major or minor scales. But above all, they have a buzzing power, which suits well to Irish music. Sometimes we call them "power chords".
​
Chords building reminder:
modal: 1 + 5
major: 1 + 3 + 5
minor: 1 + 3 minor + 5
7th: 1 + 3 + 5 + 7 minor
7th major: 1 + 3 + 5 + 7 major
Sixt: 1 + 3 + 5 + 6
Sus4: 1 + 4 + 5