In the beginning

Here's a basic look at chords, how they relate to a key, how they're built and what are their names.
I have seen the term chord defined in two ways:
1 - a chord equates to 'more than one pitch sounding simultaneously' - so any two or more notes sounding together constitutes a 'chord'
2 - a chord equates to 3 or more notes sounding simultaneously.

I personally was taught the first definition but having looked through many sources, the second definition is more widely accepted
2 or more notes? 3 or more notes? It really is note that important and not worth the fuss of deep debate. The important thing is that when notes sound together the make chords – as opposed to a note sounding alone.

First we will look at 'triads' which are chords made up of three notes.
The three notes are a 'root', a '3rd', and a '5th'.
The root is the note from which the other notes are calculated.
The 3rd and the 5th provide the tonal characteristics of the chord and ultimately provide the chords 'name'
This is where we get back to using intervals again.
A 3rd can be either a minor or major interval from the root [no different from working out the 3rd interval in a scale]
A minor 3rd is 1 1/2 tones [3 frets] above the root
A major 3rd is 2 tones [4 frets] above the root
Obviously you can't play two notes at a time on a single string so this is purely a means of working out what the note value is.

A 5th can be either 'perfect' or 'imperfect'.
If it is perfect then it is 3 1/2 tones [7 frets] higher that the root.
If it is imperfect it can be either 'diminished' or 'augmented'. Think about what these words actually mean.
To diminish something is to reduce it.
So a diminished 5th is a semi tone flat from the perfect 5th which is therefore 3 tones above the root [6 frets].
Likewise, to augment something is to add to it so an augmented 5th is a semi tone higher than a perfect 5th which is 4 tones [8 frets].

A major key contains a 7 note scale – we've seen this already.
Each of the scale can be the 'root' note of a chord as well as simply a note in its own right.
This means that there are 7 chords in a specific key, each build upon the root of the notes from the scale.
Chords are numbered in 'roman' numerals.
So if you see:
I, II, III, IV, V, VI, VII - then these correspond to the 1st, 2nd, 3rd etc chord of a given key.
The name of a chord is determined by the note value of the root and the interval of the 3rd and the 5th.
A 3rd can be either minor or major and is therefore crucial to the name of the chord.
If the 5th is 'perfect' then it is considered to be pretty normal and is therefore not mentioned in the name - so you only describe 5ths if the are imperfect.

Let's work out a few of the chords in the key of G major - then all will become clear.
The 1st degree [note] in the key of G is, funnily enough - G
So the root of chord 'I' in the key of G is also G.
Next we need to find the 3rd of this 'G - something' chord.
The notes in the key of G major are: G, A, B, C, D, E, F#
We find the 3rd from the scale by counting up from the note that represents the root [including the root].

1st--2nd--3rd
G....A.....B
the 3rd is therefore the note B
by working out the distance of B from the root of G we can then figure out what type of 3rd it is [major or minor]
the interval turns out to be 4 frets higher than the root
from this we can now tell that it is a major 3rd
So far we have the chord 'G major - something' so now it's time to look at the 5th.
1st--2nd--3rd--4th--5th--6th--7th
G.....A.....B....C....D....E....F#
looking at the scale shown above and counting 5 notes from the root [including the root] we see that the 5th is the note D
we can now find out the interval between the root and the 5th
the note D is 7 frets higher than the root of G so it is 'perfect' and therefore will not be included in the name of the chord
We have now just calculated chord I [spoken: "chord one"] of the key of G
the chord is named 'G major' and when writing chord charts it is represented as G

1st--2nd--3rd--4th--5th--6th--7th
G.....A.....B....C....D....E....F#
Moving on, the 2nd degree of the G major scale is the note A so chord II will be 'A something'...
To find the 3rd of this 'A - something' chord we must go back to the scale by counting from the note that represents the root [in this case, from the note A]
1st--2nd--3rd
.A.....B......C
the 3rd is therefore the note C
by working out the distance of C from the root of A which happens to be 3 frets above the root, we can now tell that it is a minor 3rd.
So far we have the chord 'A minor something'. Time to look at the 5th.
1st--2nd--3rd--4th--5th--6th--7th
.A.....B....C.....D......E.....F#...G
and the 5th is the note E
we can now figure that the note is 7 frets higher than the root of A so it is 'perfect' and therefore will not be included in the name of the chord.
We have now just calculated chord II [spoken: "chord two"] of the key of G.
the chord is named 'A minor' and when writing chord charts it is represented as Am.
So, get the brain going and calculate the remaining chords that are diatonic to the key of G major.

To help you, here is a list of the names of different types of some triads and how they are 'spelt' [what their intervals are]
Another little tip - one of them you won't use but I'll let you find that out for yourself

X major [written X] = root, major 3rd, perfect 5th
X minor [written Xm] = root, minor 3rd, perfect 5th
X major augmented 5th [written X aug5 or X #5] = root, major 3rd, augmented 5th
X diminished [written X dim or Xº] = root, minor 3rd, diminished 5th

Lastly, usual stuff: applies - shout if you're stuck, confused or want a beer……..

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Frank